The fun part came months ago. You cashed the tax “rebate” check that landed in your mailbox.

But now it’s tax-filing season and this supposedly simple rebate  a provision of last summer’s $1.35 trillion tax cut  already is the No. 1 error among early filers.

The Internal Revenue Service warns that some taxpayers are leaving the relevant box blank and are missing out on a tax break. Others mistakenly are jotting down what they cashed. A few are worried they must give back the money.

And then there are the people who didn’t hear that the government belatedly reversed course and is giving the tax break to dependents, too  but then steers them through an entirely different process to claim it.

Even though there are relatively few tax law changes to bone up on this year, don’t let down your guard. The IRS figures you’ll need 13 1/2 hours to complete your 1040 form  a half-hour more than last year. When lawmakers maintain the status quo, it means they’re maintaining the status confused.

When the good times were rolling, maybe you could afford to be sloppy. No longer.

“Don’t you want to be extra careful at a time when layoffs are looming, the economy is slowing and money is tight?” asked Sharon Kreider, a certified public accountant.

Let’s start by clearing up one bit of confusion about the “rebate” right away. It wasn’t a rebate. The IRS originally called those 86 million checks an “advance payment” of the savings from the creation of a new 10 percent tax bracket in 2002. But now the IRS has muddied things further by calling it the “rate reduction credit” on your 1040.

Whatever you call it, it’s a nice chunk of change. That 10 percent bracket applies to your first $6,000 of income if you’re single, $10,000 if you are head of household, and $12,000 if you file a joint return. That adds up to $300, $500 and $600, respectively.

Part of the confusion stems from the government’s decision to give you a break on your 2001 taxes months before you’d actually file your tax return. To get the checks out and stimulate the economy last summer, the government based your credit on your 2000 tax return.

That leaves us with four basic paths of advice:

 If you got the maximum check: Case closed. Do nothing. Don’t report it as income on either your federal or state return. If you immediately spent the cash, pat yourself on the back for patriotically goosing the economy.

 If you didn’t get the full amount: First, don’t assume the IRS goofed. Nearly 96,000 checks were returned, often because taxpayers moved without leaving a forwarding address. But sometimes the IRS kept cash to pay down such things as back taxes, child support and unpaid student loans.

But if you didn’t get what you are entitled to, complete the worksheet and enter the result on the line for the rate reduction credit. That’s on line 47 of the 1040 form, line 30 of the 1040A, and line 7 of the 1040EZ.

 If you got more than you deserved: This can happen when you qualified based on your 2000 tax return but wouldn’t qualify in 2001. Say, for example, that your child went off on her own in 2001, costing you the head-of-household status you enjoyed in 2000. Technically, you should have gotten $300, not $500.

It doesn’t matter.

“Everyone who got one gets to keep it, regardless,” said Martin Nissenbaum, a national director for Ernst & Young. “You never have to give it back.”

 If you were a dependent in 2000: The 2001 tax cut originally excluded dependents, nonresident aliens and estates and trusts. That meant that children who flipped burgers after school didn’t benefit from the 10 percent bracket like their parents.

Fortunately, lawmakers later reconsidered, at least for dependents.

To get what’s coming to you, you must file a tax return. But ignore the rate reduction credit and its worksheet. Instead, use the “Tax Computation Worksheet for Certain Dependents.” Jot the result on line 40 of the 1040, line 26 of the 1040A or line 11 of the 1040EZ.