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April 15 is just around the corner — Monday, to be exact. For many people, it can be a time fraught with anxiety. Here’s how to handle six of the what-ifs most likely to keep you up at night.

What if I need more time? File an extension. It’s IRS form 4868, and you can do it online. That gives you until Oct. 15 to file your federal income taxes. If you don’t, a failure-to-file penalty kicks in, which is 5% per month of any taxes you owe.

But (yes, there is a “but”), it’s only an extension of time to send in your paperwork, says Mark Steber, chief tax officer at Jackson Hewitt Tax Service. If you’re getting a refund, you don’t need to worry. But if you owe or even if you think you might owe, you’ll have to do some number-crunching now, because 90% of your estimated tax liability has to be paid by April 15.

Just taking a guess isn’t really a good idea, Steber says. “You can guess, and many people do, but you have to guess in the IRS’s favor. You have to overestimate, not underestimate.” If the IRS doesn’t get paid what you owe them by the 15th, a late-payment penalty is applied. This is 1/2% per month on your outstanding tax bill.

If you’re self-employed or in another situation where you pay estimated quarterly taxes, you could also get dinged with an additional 1/2% per month penalty fee for underpaying those.

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What if I owe more than I can pay? The IRS has a few options for people who are surprised by a big-ticket tax bill. You have to be pro-active, though. Reaching out to the IRS, even if it’s to say, “Hey, I can’t pay you right now,” is much better than waiting for them to initiate contact.

One option is a short term extension. You get up to 120 days, and you’ll have to pay the late payment penalty plus interest. The IRS ties its rate to the cost of federal borrowing; right now it’s 3%.

If you’re going to need more time, start an installment plan. If your tax bill is less than $50,000, you can do this online. You’ll pay the same interest and penalty rates as people who get a short-term extension, and there’s a one-time fee of $105 to set up the installment plan ($52 if you have your payment direct-debited out of your account every month).

You can also pay with a credit card, but there are a couple of drawbacks to that. The interest on your credit card is almost certainly higher than what the IRS charges under a payment plan, and there’s also a convenience fee of roughly 2% charged by the companies the IRS uses as a middleman to process credit card payments.

If you’re really struggling financially — say you declared bankruptcy last year — give the IRS a call. “If you have a hardship, cotnact the IRS,” says Jackie Perlman, principal tax research analyst for the Tax Institute at H&R Block. “They’ve shown willingness to work with people, but you have to talk to them.”

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What if I’m missing an important document like a W-2? First, try contacting your employer or the organization that issued you the form. A lot of workplaces now have systems for accessing W-2s online, so it might not be too late to replace that envelope the dog shredded or your spouse chucked in the recycling bin by mistake.

If you can’t get the document (for instance, if you worked for a company that went bankrupt), IRS form 4852 acts as a proxy, letting you fill out — to the best of your knowledge — what’s on that missing document. In the case of a W-2, your last pay stub of the year should help you come up with a number.

If it’s too hard to estimate, Perlman says pay any other tax you know you owe and file for an extension until you can get another copy of the missing document.

What if I didn’t keep track of my deductible purchases last year? If you live in a state where you deduct sales tax paid in lieu of income tax, the IRS has tools to help give you an estimate. Steber says most taxpayers, though, are better off trying to reconstruct the spending that would qualify for tax breaks, especially if you made a qualifying big-ticket purchase last year. Track down receipts, credit card statements, or bank statements. “The stronger the documentation, the better,” he says.

What if I already filed, but I forgot to include something? You have up to three years to amend your tax return and add a deduction, Steber says. Use form 1040X (the IRS goes into detail on its amended return information page here). The statute of limitations is three years, so April 15 is the last day you can file an amended return for your 2009 taxes. Amended returns do take longer to process — Steber says expect to wait about six weeks, since the IRS does look at amended returns more closely.

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Steber says it’s a common urban legend that filing an amended return will put you in the IRS’s cross hairs for an audit, though. “In most cases, the amended return is asking for money, and that’s the reason for the additional scrutiny,” he says.

What if I didn’t file taxes at all last year — or longer than that? If you didn’t file and are due a refund, you have a three-year statute of limitations to claim that refund, Perlman says. If you haven’t done your taxes for years and you know you’ll owe money, that’s a little trickier. In general, the IRS can go back three years, Perlman says, but they can look back at what you made and what you should have been paying for six years if you underreported your income by 25% or more.

Start by doing this year’s taxes and paying anything you owe. After this year’s, start with the first year you didn’t pay and work your way forward, Perlman says. There could be something in one year’s return that carries forward to the subsequent year.

If the last time you paid taxes was before the iPhone was invented, Perlman suggests getting help from a tax pro to sort through the mess and help you reconstruct your financial records.