In the Gregorian calendar that is commonly used these days, we are now early in the year 1988. The Islamic calendar identifies this year as 1408. In the Chinese calendar, this is the Year of the Dragon. According to the Jewish calendar, it is now 5748.

But for the Internal Revenue Service, which uses the Gregorian calendar but runs its individual income tax collection system on an April 15 fiscal year, this is Year One of the post-tax reform era.

The Tax Reform Act of 1986 is perhaps the most important piece of tax legislation since the the modern income tax was introduced in 1913. And it came right on the heels of a string of major tax changes: the Economic Recovery Tax Act of 1981, the Tax Equity and Fiscal Responsibility Act of 1982, the Social Security Amendments of 1983 and the Deficit Reduction Act of 1984.

To complicate the situation even more, additional tax legislation was enacted on Dec. 22, 1987. Much of this new legislation pertained to business, and some of the changes simply modified or corrected problems that appeared after the new tax law was enacted. Many items are effective beginning in 1988. Those that apply to 1987 individual tax returns are not specifically identified here; instead, the correct instructions for 1987 are included regardless of whether they result from the 1986 law or the 1987 act.

One of the principal advertised goals of the 1986 law was tax simplification. Does the new tax system live up to its advance billing? Well, there are winners and losers. Because of the increases in personal exemptions and the standard deductions, as many as sixmillion people who filed tax returns for 1986 will not have to file for 1987 and future years -- clear winners all.

Millions more who have been itemizing deductions will no longer do so because of both the higher standard deductions and the limitations on many of the itemized deductions previously allowed on Schedule A. And those very limitations will simplify tax return preparation; elimination of the deduction for sales tax, for example, means one whole category of deductions that you can forget about.

But for many taxpayers, the chore of preparing their tax returns will be made more difficult by the new rules, some of which in turn require new calculations and even new forms. And the whole process is further complicated by the fact that several of the changes are being phased in gradually over four years, which means that new elements introduced for 1987 will be further modified in 1988 and succeeding years.

It will be 1991 before some of the changes mandated by the 1986 law are in full force; and that's only if -- a very big "if" -- no further tax legislation is enacted between now and then.

The Internal Revenue Service, faced with literally hundreds of changes in the tax laws, has attempted to keep the taxpayer's job as simple as possible. Those forms that continue in use, such as the basic 1040 and Schedules A and B, look pretty much as they have in the past.

But don't be lulled into complacency by the familiar appearance; a number of line-item changes were mandated by the new law and, of course, the addition of several new forms simply adds to the volume and complexity of calculations needed.

Nevertheless, if you have been completing your own tax returns in the past, you should be able to do so this year. But don't make any assumptions about the procedures; study this Tax Guide carefully, along with the IRS instruction booklet and any special publications you may acquire.

Even if you use a commercial preparer, you must provide all the basic data. To do that, and to make it possible for the preparer to provide a good final product, you should be familiar with the rules governing income, exemptions, deductions, exclusions and credits, and particularly with the myriad changes enacted since you sat down to work on your 1986 return.

Above all, get an early start. If you intend to tackle the job yourself, last year is not a valid guide -- you will need to spend substantially more time reading the rules and studying the forms. Free help from the IRS is available, but expect the phone lines and walk-in offices to be much busier than in previous years.

You can count on the IRS assistance people having troubles, too. They may have to keep you waiting, or put you on a call-back list, while they consult a backup expert in whatever area your question involves.

If you decide you need professional help this year, remember that the pros are likely to be swamped, too. You may have difficulty getting a qualified person to accept you as a client at the last moment -- another reason for not delaying.

Whether you use a preparer or complete your own return, you have a legal responsibility to pay every dollar of tax due.

On the other hand, you have a right to reduce your tax liability to the lowest amount possible under the law. Your responsibility doesn't extend to overstating your taxes by reporting nontaxable income or omitting deductions or credits for which you qualify.

Let's get to the work at hand. We offer The Washington Post Tax Guide XVI, covering the tax forms and schedules most used by our readers; we will try to anticipate the most commonly asked questions, and point out the many areas that have been changed from previous years.

Following the sections on the federal tax rules, you will find special sections on Maryland, Virginia and the District of Columbia, with particular emphasis on those areas where state requirements differ from federal instructions.

Do It Yourself? Unless you have complicated financial dealings, you should be able to handle your own tax return.

Income from wages or salary, a pension or retirement plan, interest and dividends, a few stock transactions, even a limited partnership are handled in a straightforward manner and should present no problems.

With the help of this Tax Guide and the pertinent IRS publications, you should be able to make it.

In addition to the instruction booklet that accompanies your tax forms, pick up a copy of IRS Publication 17, "Your Federal Income Tax," free at local IRS offices or by mail from the address given in your booklet. If you operate a business, Publication 334, "Tax Guide for Small Business," will prove helpful.

Taxpayers who are more fluent in Spanish than English should get a copy of IRS Publication 579S, "Como Preparar la Declaracion Impuesto Federal." Publication 850, "English-Spanish Glossary of Words and Phrases Used in Publications Issued by the IRS," might also be useful.

If you need personalized help or you're looking for an answer to a specific question, you can get advice or assistance from the IRS, either on a walk-in basis at your local IRS office or by phone over special assistance lines.

Residents of the District or Montgomery or Prince George's counties should call 488-3100 for help. Northern Virginia is served by the Baileys Crossroads office at 557-9230.

To request forms or publications (but not for answers to questions or other help), call the IRS at 1-800-424-3676 from 8 a.m. to 8 p.m. Monday through Friday, from 9 a.m. to 3 p.m. on Saturday.

Hearing-impaired persons who have access to TTY equipment can get assistance from the IRS by calling 1-800-428-4732 any day between 8 a.m. and 6:45 p.m. until April 15 and from 8 a.m. to 4:30 p.m. after that.

The IRS also provides recorded tax information on a wide variety of subjects. "Tele-Tax" is available from both push-button and rotary-dial telephones. Residents of the District and local Marylanders and Virginians can call the D.C. number: 628-2929. Other taxpayers in Maryland and Virginia should use 1-800-554-4477.

IRS tax instruction booklets contain a list of about 150 Tele-Tax subjects, along with instructions on how to use this service.

Generally you will get good information from the people at the local IRS offices. But the complexities this year, compounded by the use of temporary help with limited training during the height of the tax season, may reduce the quality of the advice. You should do as much research as possible on your own before going to the IRS, to help you understand what you are told and to be in a position to question further any information that doesn't seem right to you.

In a change from previous years, the IRS will waive tax penalties for taxpayers who paid too little in 1987 because the IRS provided incorrect answers to their tax questions over the telephone. However, you will have to prove that you prepared your tax return in accordance with information provided by IRS officials on the other end of the telephone line, probably by citing the name of the person you talked to and the day and time of the call.

Use a Pro? If you decide you want to use a professional preparer, you have a fairly wide range of choices. A "tax preparer" might be a skilled accountant for the government or private industry who moonlights at tax time. But he or she may be just someone who has read a tax book or has taken a short course in tax preparation. There are no federal or state rules establishing standards of training or experience -- anyone can hang out a shingle as a tax preparer.

The large chain operations, most of which now provide computerized service, generally give satisfactory service at low cost if you have a routine tax situation.

Some of the chains offer a more sophisticated tax preparation service at higher cost, sometimes in your own home or office. But they may not be able to handle properly a complex return or unusual situation.

If you have that kind of problem, you may want to visit a public accounting firm or tax attorney. Their fees are usually higher, but their knowledge -- and resource materials -- may be more extensive than you will find at most tax offices.

Another high-level tax preparer is the "enrolled agent." Although he is neither an attorney nor CPA, the enrolled agent has passed a comprehensive examination given by the IRS; he may not have a broad background in general accounting or law, but you can expect him to be very competent in tax matters.

Whatever the source of your help, the preparer or parent firm should be around all year to assist you in answering queries from the IRS. The person who prepared your return may accompany you or represent you at an audit, but only an attorney, CPA or enrolled agent may represent you at appeal levels or in the tax courts.

Some quick tips: Stay away from anyone who tells you what you can "get away with" or who promises a refund before examining your tax information.

Avoid a preparer whose fee is based on a percentage of your refund or of the "tax savings" he has found for you.

Don't sign a blank form or a return that has been filled out in pencil, and never agree to have a refund sent to the preparer rather than to you.

You are responsible for your return even if it is prepared by another, so be sure you understand -- and agree with -- every entry. If the preparer cannot or will not answer your questions and explain the entries, go elsewhere.

Be sure the preparer signs the return and enters his tax identification number. He is required to give you a file copy of the return when the original is offered for signature.

You should know that both civil and criminal penalties may be assessed against both the preparer and the taxpayer for tax deficiencies caused by carelessness in gathering and entering data and for material omissions, even if unintentional.