The Washington Post's annual Tax Guide is a step-by-step explanation on filling out your own federal and state returns. The eight articles will run on consecutive days through next Sunday.

Liability for filing a federal income tax return for 1979 is determined by a combination of total taxable income, age and marital status. The gross income minimums for filing are higher for 1979 than they were for 1978.

These new minimums for the various filing categories, and the normal number of $1,000 personal exemptions which apply in each case, are shown in the accompanying table.

If you are entitled to a refund as a result of the earned income credit, you should file even if you had no tax liability and paid no taxes during the year. This credit, available to certain taxpayers with annual income of less than $10,000, is explained in an upcoming article.

If during 1979 you received advance payments of the earned income credit from your employer, you must file a return regardless of the amount of your income.

You should file a return to claim a refund of income tax withheld from your pay if you are not otherwise required to file.

Any taxpayer may use Form 1040, the long form. You should use the short form 1040A if all three of the following apply:

Your 1979 income did not exceed $20,000 ($40,000 on a joint return).

Your income consisted only of wages or other employe compensation and $400 or less of either interest or dividends.

You do not itemize deductions.

You must use Form 1040 if your adjusted gross income exceeded the $20,000-$40,000 ceiling; if you had pension or annuity income, capital gain dividends or income or loss from rental property, a business, estate, or trust; or if you had received more than $400 in either interest or dividend income.

(If you have no tax liability and file only to claim the earned income credit, you may use the short form even if you received more then $400 in interest or dividends.)

Form 1040 also is required if you itemize deductions; claim any "adjustments to income" such as moving expenses, employe business expenses, an IRA or Keogh investment, alimony payments, or a disability income exclusion; or if you are entitled to the credit for the elderly, for child or dependent care expenses, or for residential engergy expenditures.

And Form 1040 must be used if you compute your tax by income-averaging; if you paid estimated tax for 1979 or wish to apply any part of your 1979 refund to 1980 estimated tax liability; or if the total number of your exemptions exceeds the highest number given in the tax table.

Finally, you must use Form 1040 if you may be claimed as a dependent on your parent's return, had $1,000 or more in unearned income (such as interest), and had earned income of less than $2,300 ($1,700 if married filing separately); or if you file as a "qualifying widow(er)," as explained later in today's article.

If you were married on Dec. 31, 1979, you are considered by the Internal Revenue Service to have been married for the entire year. If you were unmarried (single, divorced or legally separated) on that date, then for income tax purpose you are considered to have been single for all of 1979.

In this case, if the total of itemized deductions (line 39 of Schedule A) is less than the ZBA, you must perform a special calculation in Part II of Schedule TC. See page 12 of the instruction booklet.

If you were married but living apart from your husband or wife during the entire year and meet both of the following tests, you may file a separate return as a single individual:

You paid more than half the cost of maintaining your home during 1979; and

Your home was also the principal home of your dependent child for more than six months of the years.

If the above applies to you, you may qualify to file as head of a household; check the requirements given later in this article.

If your husband or wife died during 1979 and you had not remarried by Dec. 31, you may file a joint return and claim both your personal exemption and the exemption for your deceased spouse.

Sign the return yourself, then enter "Filing as surviving spouse" under your signature. Enter the date of your spouse's death in the name and address space at the top of the form.

If you are due a refund on the joint return, you must also file Form 1310 with the return to establish your right to the refund. Do not send a copy of the will or death certificate with the return.

If an executor or administrator has been appointed for the estate, he may have filed (or may intend to file) a separate "final return." In any case, the estate tax return may have an impact on your income tax return. If the executor is not himself a professional, you are likely to need help in this situation.

If your husband or wife died during 1977 or 1978 and you had not remarried by the end of 1979, you may be eligible to file as a "qualifying widow(er)" -- and use the joint return tax rates -- if you meet all three of these tests:

Your were eligible to file a joint return in the year of your spouse's death (whether you actually filed that way or not);

Your home was the main home during 1979 of a child whom you claim as a dependent; and

You paid more than half the cost of maintaining that home for the entire year.

If you qualify, you may not claim the personal exemption for your deceased husband or wife, even though you are using the joint return tax rate. The personal exemption can be taken only if death occurred in 1979.

If you were single on Dec. 31, you may qualify for filing as "head of a household" -- and the lower tax rate that goes with it -- if you meet any of the following tests:

You paid more than half the expense of maintaining your home which was also the principal home all year of your unmarried child, stepchild, foster child, or grandchild (whether or not the child qualifies as your dependent); or

You paid more than half the cost of upkeep for your home which was also the principal home of any other relative you claim as a dependent (except a dependent under a mutual support agreement); or)

You paid more than half the cost of maintaining a home which was the principal home of your dependent mother or father (or both), even if you did not live in it yourself.

A taxpayer filing as head of a household does not get any extra personal exemptions, but gets a lower tax rate than other single taxpayers.

If you file as head of a household, enter on line 4 the name of the person who makes you eligible for that status. If there is more than one such person, enter one name only. Exemptions and Dependents

In addition to the basic personal exemption for each taxpayer, a person who is 65 or older or legally blind may claim an extra $1,000 exemption. But the additional exemption is available to the filing taxpayer(s) only; it may not be claimed for a dependent. If your 65th birthday was Jan. 1, 1980, you may take the extra exemption on your 1979 return.

If you claim the additional exemption for blindness, each year you must attach a statement that you are completely blind, or a statement from your physician or optometrist that your vision with glasses is no better than 20/200 or that your field of view is no greater than 20 degrees.

If your physician or optometrist states that in his opinion this eye condition will never improve, you need only attach the statement once. If following years enter on your return: "See statement attached on return for (year)."

There is no change this year in the five tests to determine if a person qualified for a dependent exemption on your tax return.

Citizenship or residency test. A dependent must be either a U.S. citizen or have resided in the United States, Canada or Mexico during 1979. (This test is waived for an alien child adopted by and living with a U.S. citizen in a foreign country.)

Relationship or member of household test. A relative (as defined on page 8 of the IRS instruction booklet) need not have lived in your home to qualify as a dependent. If you file a joint return, a dependent meets this test if related to either spouse.

Once this qualifying relationship has been established, it does not end because of death or divorce. For example, if your husband or wife has died, you may continue to claim your former father-in-law as a dependent even if you have remarried.

A dependent who is not related must have lived in your home for the entire year. Absence at school or for hospitalization is not disqualifying if your home was his home when not away.

Income test. To qualify, a dependent must not have received $1,000 or more in income during 1979 -- but you need count only income which is subject to tax. Do not include, for example, bona fide gifts, Social Security payments or Veterans Administration benefits; interest on tax-exempt securities; or scholarship payments.

The income test is waived for a child who was under the age of 19 on Dec. 31, or who was a full-time student (regardless of age) during any five months of the year.

There are limitations on the deductions that can be claimed on the tax return of a child who is eligible to be claimed as a dependent on the tax return of another. The applicable rules are found on page 12 of the instruction booklet.

Support test. In order to claim an exemption for a dependent, you must have provided more than half of his support during 1979. Unlike the income test, all income of the dependent (except scholarships), whether taxable or not, must be considered in determining how much the dependent contributed to his own support.

However, only as much of that income as was actually spent by the dependent on items of support -- that is, on necessary living expenses or on capital purchases such as a car -- need be included in the calculation.

Do not count as support any money your dependent deposited in a savings account, unless he withdrew it later in 1979 and then spent it on his own support. Any money your dependent paid for life insurance premiums, income tax payments or Social Security contributions should also be excluded.

Include in you contributions to the support of the dependent the cost of living expenses (lodging, food, clothing, medical care, education) and of capital items such as a car, furniture, or a television set bought for his or her own use (but not if bought for the household).

If you provided separate living quarters -- such as an apartment for a dependent parent -- count the cost to you of that housing. But if your parents lived rent-free in a house you owned, use the fair rental value (based on comparable houseing in the same area) rather than the cost.

In this situation you are not entitled to claim depreciation of other normally associated with rental property; but you may include property taxes, mortage interest, and casualty losses as itemized deductions on Schedule A.

Joint return test. Normally a married person who files a joint return with his or her spouse may not be claimed as a dependent by another taxpayer (except in the case of a joint return filed solely to obtain a refund of taxes withheld).

However, you may claim an exemption for a married dependent who meets the other test if the dependent's spouse files a separate return and doesn't claim an exemption for your dependent.

As a general rule, a divorced parent who had custody of a child for the greater part of the year may claim the child as a dependent. But the other parent may take the exemption if the decree of divorce or separation agreement says so and he or she contributed $600 or more to the child's support during 1979.

In the absence of a specification in the agreement, the "non-custodial" parent may claim the exemption if he or she provided at least $1,200 for support and the parent having custody cannot prove a larger contribution to the child's support.

In adding up the amount of support contributed, a parent who has remarried may include support furnished by the new spouse.

Several taxpayers may jointly support a dependent (brothers and siters caring for a dependent parent, for example) with no single person contributing more than half the total support.

If together you contributed more than half of the support and the dependent otherwise qualifies, one of the group may take the examption if he or she provided at least 10 percent of the total support and the others agree to waive their claims.

Each year a different taxpayer may claim the exemption, so it may be rotated among the several contributors from year to year. The taxpayer claiming the exemption must attach to the return a separate Form 2120 (agreeing to the arrangement) for each other qualified contributor to the total support.