The basic rules for filing an individual federal income tax return for 1985 are unchanged from 1984 requirements. Your liability for filing a return is determined by a combination of your age, marital status and taxable income.

However, because 1985 saw the beginning of "indexing" -- that is, modifying the various dollar figures by the amount of increase in the cost-of-living index -- the numbers determining your liability to file a tax return have changed.

The accompanying table lists the new income floor for each category of taxpayer. If your income equals or exceeds the amount shown for your filing status, you are required to file a federal tax return. (State requirements may differ.)

There are some special rules. Even if your income falls below the specified minimum, you must file a return under any of the following circumstances:

*To claim a refund of income tax withheld from your pay.

*If you are entitled to a refund as a result of the earned income credit.

*If you received advance payments from your employer in anticipation of qualifying for an earned income credit.

*If you are eligible to be claimed as a dependent on your parent's return and you had taxable interest, dividends or other unearned income of $1,040 or more.

*If you had net income of $400 or more from self-employment. (This number is not indexed; the amount is established by Social Security regulations rather than IRS rules.) How to File

Your federal income tax return for 1985 must be in the mail and postmarked not later than midnight Tuesday, April 15. If you can't make that deadline, don't simply ignore it; penalties may be assessed for failure to file or to pay your tax on time.

By filing IRS Form 4868 by April 15, you can get a four-month extension, to Aug. 15. But that extension applies to filing only, not to the payment of any tax due. You must estimate your eventual tax liability on Form 4868 and pay any expected deficiency when you file the request for extension.

If you take advantage of the extension period, then later when you do file your return you must use Form 1040 or 1040A; you may not use 1040EZ and may not request that the IRS compute your tax.

Your request for extension will not be acknowledged by the IRS; if the 4868 has been completed properly, any payment due is included and it is filed by April 15, then approval is automatic. But when you do file, attach a copy of the completed Form 4868 to your tax return.

If you're out of the country on April 15, you get an automatic extension to June 16 (the 15th falls on Sunday) without having to file Form 4868 or submit any written request at all. But when you file your return later, attach a written statement explaining the circumstances that led to the delay. In addition, you must pay interest on any tax due from April 15 to the date of filing and payment.

Unless you have asked the IRS to figure your tax, you should attach to the return any payment due. Do not send cash. Attach a check or money order, payable to the Internal Revenue Service, to the front of the tax form where indicated. Be sure to write your Social Security number on the payment so it can be identified if it becomes separated from the return.

Use the mailing envelope that came with your tax package. If you don't have it, send the return as follows:

*A resident of the District of Columbia or Maryland using a Form 1040 accompanied by payment for a balance due should mail the return to Internal Revenue Service Center, Philadelphia, Pa. 19255-2222.

*District and Maryland residents filing any of the three forms without payment or requesting a refund should address the return to Internal Revenue Service Center, Philadelphia, Pa. 19244-4444.

*Federal returns of Virginia residents on Form 1040 with a balance due go to Internal Revenue Service Center, Memphis, Tenn. 37501-2222.

*Virginia residents filing 1040, 1040A or 1040EZ without payment or seeking a refund should send the return to Internal Revenue Service Center, Memphis, Tenn. 37544-4444.

If you received a forms package from the IRS, use the peel-off label from the booklet on the return. Make any necessary corrections to the information on the label. If you go to a professional preparer, take along the booklet, or at least the cover page with the label.

This label does not contain any special coding to flag your return in any way. Use of the label is not mandatory, and you will not be penalized for failure to use it. But it permits the IRS to make optimal use of its expensive automated equipment, saving employe hours and taxpayer money.