Does the sight of a tax form make your stomach turn?

Simpler or more complex? The answer for taxpayers wrestling with their tax forms this year is both.

More people are now eligible to use a briefer federal form, but in a new twist for 2004, same-sex married couples in Massachusetts will have to file as singles on their federal returns and as married on state returns.

It's the federal return that gets the most attention, and you can't rely on what you claimed a year ago.

You'll cheat yourself, for example, unless you deduct 2004's inflation-adjusted $3,100 exemption for dependents and yourself -- up $50. Children under 14 can have earned up to $1,600 in investment income in 2004 before being

taxed at their parents' tax rate, up from $1,500 for 2003.

And your eligibility for some breaks may have changed. On 2003 returns, itemized deductions were reduced when adjusted gross income exceeded $139,500. For 2004 that doesn't happen until income tops $142,700.

Understanding the rules may not be enough. Many investors know that capital gains and most stock dividends are taxed less by Uncle Sam than other income. ''But working through the calculations on the form is a lot more complex than the concept itself," says Edward Smith, a tax partner in the Boston office of accounting firm BDO Seidman.

But the payoff from doing the capital-gains worksheet is that profit from the sale of an asset owned longer than a year and many dividends will be taxed at either 5 percent or 15 percent instead of at normal rates as high as 35 percent.

Procrastinators have some leeway. The Internal Revenue Service will extend the filing deadline to Aug. 15 for almost anyone who asks. Form 4868 gives the details and also explains how to get a last-minute extension by phone. An extension protects you against late filing penalties, but not from interest or possible penalties on tax you owe.

There's no penalty for filing late without an extension if you are due a refund -- which nearly eight of 10 taxpayers get. (Last year's average was $2,126, according to the IRS.)

Early birds who already filed their returns can still check for missed deductions or other goofs. They can file an amended return using form 1040X to correct a mistake. It's best to do that by April 15 to avoid penalties or speed up a bigger refund, but you generally have three years in which to file an amended return.

Even people who pay someone to do their return should keep up with the rules, as a guide to the financial data they must gather and as a check on the preparer's work. Here are some other pointers on your federal returns:

Keep it simple. Instead of plowing through the 1040 form, more people can now use the abbreviated 1040A or the single-page 1040EZ. The income ceiling for using the 1040A or 1040EZ has been doubled from $50,000 to $100,000 of income after deductions.

But there are other limitations. You must, for example, be under 65 and can't claim dependents to use the 1040EZ and neither the 1040EZ or 1040A can be used if you itemize deductions or have gains or losses from selling investments.

Self-employed people and employees with a sideline may be able to use a simplified C-EZ form to report their business income and expenses. The IRS for 2004 will let people claim up to $5,000 in general expenses on the C-EZ, double the ceiling for 2003.

Tip: Itemizing deductions allows you to deduct mortgage interest, donations, and state and local tax, but many people find that the fixed standard deduction that requires no receipts works fine for them. On 2004 returns, the standard deduction is $9,700 for a couple filing jointly and $4,850 for a single person. A couple 65 or older gets $11,600; a single senior gets $6,050.

Getting an education. Choosing among the write-offs for college bills takes concentration but can shave a parent's tax bill. The Hope credit for the first two years of school or the Lifetime Learning credit for any year is generally the first choice. The Hope can save up to $1,500 in tax and the Lifetime credit can save up to $2,000.

But people barred in whole or part from a credit because their 2004 gross income exceeds a limit ($42,000 on a single return; $85,000 on a joint return) may be eligible for an alternative tax deduction of up to $4,000 for tuition and fees. The deduction, though, saves less tax than the credits -- a maximum of $1,000 for someone in the 25 percent tax bracket.

Tip: Teachers who pay for classroom supplies can deduct $250 of the cost even if they don't claim itemized deductions -- a break extended through 2005.

Charity's reward. Don't shortchange yourself on this popular itemized deduction -- averaging about $3,610 on 2003 returns.

In addition to cash gifts, you can deduct the value of used items you donate and expenses you incur in volunteer work, such as 14 cents a mile for using your car or the cost of a scoutmaster's uniform. Not deductible: The value of your time or raffle tickets.

The rules for 2004 donations, as before, limit a deduction to a car's market value. For 2005, generally only the amount received by a charity from its sale of the carwill be deductible.

Tip: Cash donations made in January this year to aid tsunami victims can be included on 2004's tax return.

Sales tax redux. Until the law was changed in 1986, an itemized deduction was allowed for state and local sales tax paid during the year. Meticulous types saved shoeboxes of receipts. Others simply looked up a set amount in IRS tables based on income and residence.

Now Congress has brought back the deduction for at least 2004 and 2005. But this time itemizers must make a choice.

They can claim either a sales tax deduction or a deduction for state and local income tax, with the income tax likely to be the higher amount for most people. The biggest winners are likely to be people in the states that impose a sales tax but not an income tax.

Tip: You can add to the table amount -- listed in IRS Publication 600 -- the sales tax on a car or other motor vehicle, or on home building material. That might make the deduction work for some people.

Last-minute nest egg. There may still be time to create a tax deduction for 2004 by opening an individual retirement account or contributing more to an existing IRA.

Employees not covered by a retirement plan at work or who otherwise qualify for a tax-deductible IRA have until this April 15 to make a deposit for 2004. The maximum is $3,000, but people 50 or older can deposit $3,500, which in the 25 percent tax bracket can nick $875 off their current tax liability. A couple may qualify to put in double the amounts.

Also still doable are Roth IRAs. There's no tax deduction but, unlike a traditional IRA, withdrawals are tax-free.

Self-employed people have several options, including the possibility of sinking almost 20 percent of their earnings into an IRA-like simplified employee pension (or SEP), with a top deposit of $41,000. The deposit deadline to get a 2004 deduction is April 15 or later with an extension.

Tip: You can claim an IRA deduction before you make the deposit -- as long as the deposit is made by April 15.

Don't forget the state. States generally base their income tax on the federal levy, but you have to be aware of differences. For example, in Massachusetts, long-term capital gains are not taxed at a special low rate as they are on federal returns, and depreciation deductions on business equipment may be less generous than allowed by the feds.

Same-sex married couples face complications since the federal government doesn't recognize their marriage. Same-sex couples who must file as married in Massachusetts and single on their federal returns may have to make special adjustments when reporting income and deductions.

Tip: A state-only break for commuters allows a limited deduction for highway tolls paid through the state's Fast Lane account or for spending on transit passes.

For more tax information, go to boston.com/business/taxes.