In addition to mortgage interest, points (prepaid interest), and PMI (mortgage insurance), you can also deduct your property taxes.

Home energy credits are also available, although the 2011 general credit is only worth $500 (lifetime).

If you don’t file a tax return, the IRS will calculate a substitute return for you, based on the information it knows. if you owe taxes based on this substitute, the IRS will send you a letter and a bill. If you don’t get such a letter, the IRS thinks you are due a refund (or you’ve moved and the IRS can’t find you).

Once you file your late return, if you owe the IRS, you will also owe a late filing penalty plus interest on the amount due. If you are due a refund (even if the IRS originally calculated a balance due on their substitute return), it is extremely unlikely they will charge a penalty for being late.

Just because the IRS or your state has not caught the mistake as yet does not mean they won’t.

You are legally obligated to file an amended return if you owe more tax than was shown on the original return. Common cases would include not reporting income you received, claiming deductions or credits to which you were not entitled, or claiming a filing status you were not eligible to use. You are almost always better off fixing the mistake yourself rather than waiting for someone to catch it and fix it for you.

You will owe the additional tax that should have been paid, plus interest from the due date of the return. In some cases, you might also be subject to a penalty. Make sure you check whether an amended state return is necessary as well.

If the mistake means you paid more taxes than you should have, you obviously want to amend the return to get the excess money back.

Bear in mind you have a limited amount of time to file an amended return (generally 3 years from the original due date of the return), and that some changes may affect your federal return but not your state or vice versa.

Most property you own and use for personal or investment purposes is a capital asset: your house, your car, stocks or bonds, furniture and clothing. (The tax code specifically excludes certain types of items from the definition.)

Gains on the sale of your personal use property are capital gains and are taxed as such. Losses on the sale of personal property are not deductible.

Gains on the sale of investment property are also capital gains, and losses are capital losses.

Most capital gains resulting from property held for longer than a year are currently taxed at a lower tax rate than regular income.

One important correction to Skip’s answer:

There are differences in what kind of DOCUMENTATION you need to keep to substantiate your deduction, and if you are donating non-cash items valued at more than $5,000 you generally will need an appraisal.

However, cash and non-cash donations are BOTH subject to the same limitations. Briefly:

- If you are itemizing and your total charitable contributions are less than 20% of your Adjusted Gross Income (AGI), there are no limitations on your deduction.

- Otherwise, your total contributions (cash and non-cash) is limited to 50% of your AGI for most qualified charities.

- Donations to some organizations (including veteran’s organizations and fraternal societies) are limited to 30% of your AGI.

- There are additional special 30% and 20% AGI rules for capital gain property (that is property generally held for investment, like real estate). If such a case may apply to you, see the advice of an Enrolled Agent or other qualified tax advisor.

That’s not an easy question to answer generically. There are too many variables.

A married couple with the same TAXABLE income as a single person will pay the same amount of income tax if they are in the 10% or 15% tax bracket; otherwise, the married couple will pay more (everything else being equal, of course).

On the other hand, a married couple with the same GROSS income as a single person may have lower taxable income because of the additional personal exemption they can claim.

In order to deduct investment expenses, you have to itemize your deductions (Schedule A). Investment expenses fall in the category “Miscellaneous Deductions Subject to 2% of AGI”; that is, if the total of your investment expenses and other allowable miscellaneous deductions is less than 2% of your AGI, you will get no tax benefit.

To be deductible, investment expenses must be paid related to taxable income. (If your portfolio consists solely of tax-free municipal bonds, your expenses are not deductible.) Examples of expenses you can deduct are investment management fees, IRA fees provided you pay them separately and not out of IRA funds, and a safe deposit box provided you use it to store investment related materials.

If you do your own investing, you may be able to deduct specialized subscriptions like ValueLine or investment newsletters. (Subscriptions to general finance magazines like Money and Kiplingers, or to the local paper for the stock quotes, are questionable.) If you use a personal computer to track your investments, you may be able to depreciate part of the computer cost, but strictly speaking you should keep a usage log to support that deduction.

If you buy stocks on margin, you may be able to deduct investment interest paid. This is not subject to the 2% of AGI limitation, but there are other requirements and limitations that may apply.

As a rule, sooner rather than later.

If you have had any significant changes during the year that might affect your tax situation, it’s wise to make an assessment before the end of the year. Such changes might include (this is not meant to be an exhaustive list):

- a change of job (or retiring from a job),
- a child turning 17 (ineligible for the child tax credit) or 14 (ineligible for the dependent care credit),
- a child or other family member starting or graduating from college,
- getting married or divorced,
- buying or selling a home,
- taking money out of a retirement plan

A homeowner can potentially deduct on his or her federal tax return: real estate taxes, mortgage interest, mortgage insurance (PMI), home equity interest, and points paid on a mortgage or refinance. In some cases, there are restrictions on what can be deducted. A tax professional can guide you through your particular situation.

There may also be tax credits available to homeowners, such as the energy credit, for certain energy efficient home improvements. However, the 2011 credit for many improvements is very limited. (Home appliances have never qualified for the federal energy credit.)

What you can deduct on your state taxes will depend on your state.

Regardless of what some real estate agents might tell you, most of your closing costs on the purchase of a personal residence are not deductible, even though some of the costs are taxes (e.g., mortgage recording tax).

Homebuyers who close late in the year may not have enough deductions to make itemizing worthwhile. This is sometimes a shock for new owners.

It’s not clear whether you are asking about estimating your AGI before completing your tax return (in which case George’s answer applies), or just want to know where to find your AGI on your completed tax return.

If the latter and you filed a 1040 or 1040A, look at the last line on page 1 (which says in part “This is your Adjusted Gross Income”). If you filed a 1040EZ, look at line 4.

Your marital status is determined on the last day of the year. Married taxpayers have the option of filing jointly (income and deductions combined on one return) or separately (each spouse’s income and deductions on their own return).

Filing jointly will often more result in lower tax, but not always. In addition, there are other considerations that may affect your choice filing status, such as joint liability for items on a joint return.

I overlooked your question about beneficiary designations, which do take precedence over a will. Generally, you usually want a will as well to cover any assets where you can’t specify a beneficiary, those where you forgot to specify a beneficiary, and to provide for other scenarios beyond distributing assets.

It depends on what type of stock options you have.

Statutory options (ISO) are subject to regular tax when sold. (If you exercise the options and don’t sell them in the same year, you may be subject to AMT in the exercise year.)

Stocks purchased through an ISO exercise must be held for one year from the exercise date AND two years from the grant date to receive the most favorable tax treatment; otherwise some or all of the proceeds may be taxed as ordinary income,

Non-statutory options can be taxable at grant, exercise, when vested, or when sold — depending on specifics.

When you receive options, I suggest you consult a tax advisor to discuss your specific situation.

Your marginal tax bracket depends on your taxable income and your filing status. If you look for the “taxable income” line on your return, you can look it up in a table; here’s one from Wikipedia:

http://en.wikipedia.org/wiki/Tax_bracket#Tax_brackets_in_the_United_States

Aside from the student loan interest deduction, I’m afraid there aren’t any other tax breaks for paying your student loans.

If you have sufficient itemized deductions to exceed the standard deduction, you can itemize just as anyone else can. Generally, it doesn’t make sense to itemize unless you own a house (paying mortgage interest and property taxes) and/or have a high income such that you’re paying a lot of state income tax.