"Take an unmarried couple that has a household that includes a child that is the biological child of only one of the adults," says Luscombe. "In addition, one of the adults is the sole wage earner for the household.

"Under the previous support test, the wage earner could claim head of household status. But now, using the relationship test, the exemption would apply to the biological parent in the house, who in this case has no earnings.

"And the other adult, who does have income, now loses the $3,200 exemption."

If you have a similar living arrangement and share custody with another parent or live with several adults who contribute to a child's upbringing, read the new definition carefully to make sure you and other taxpayers involved in the child's care take full and appropriate advantage of the new definition.

9. Increased IRA contribution amounts
Thanks to a tax-law change enacted years ago, contribution limits on retirement accounts have been steadily increasing. For 2005, you can put up to $4,000 into an IRA. Persons age 50 and older can contribute an added $500.

The $4,000 limit remains in 2006, but older filers this year can contribute an added $1,000.

10. Changes in the filing process
Finally, changes in ways to file your 1040 will make some computer-competent taxpayers happy, upset those who preferred a phone to a PC and delight the millions of procrastinators who don't want to think about taxes until they absolutely have to.

The IRS again is offering some taxpayers the opportunity to prepare and e-file their returns at no charge through the Free File Alliance. Nineteen software companies have made their services available at the IRS Web site, and taxpayers who make $50,000 or less should be able to find at least one company that will let them file for free.

The IRS has, however, done away with its TeleFile program that allowed taxpayers with less-complicated returns to file over the phone. These filers will now have to go back to pen and paper or try the Free File program, for which they are automatically eligible.

If you just can't get your taxes done, whether online or on paper, for free or by paying a professional, it's easier this year to delay the task. Now, instead of filing a request in April for a four-month extension and then another in August seeking two more months, the IRS says you just have to ask once.

Form 4868, used previously to make the first extension request, now will give you an automatic six-month delay. File it and you don't have to worry about your 1040 until Oct. 16.

One thing hasn't changed, though. Form 4868 will only get you more time to file your tax form. If you owe Uncle Sam money, you still have to come up with that amount or a close approximation of it by April 17 or face possible penalties and interest.

In addition to the changes wrought by these 10 laws, many pre-existing laws have had new dollar amounts added. See "Old tax laws remain, but effective amounts change."

Freelance writer Kay Bell writes Bankrate's tax stories from her home in Austin, Texas, and blogs each day on tax topics at Don't Mess with Taxes.