ongress used the new tax law to nail some people who haven't been paying all the taxes they should. Also, it stopped those who were profiteering through certain loopholes. As a result, many of you will pay more income taxes. Among the changes:

1. Some retirees don't report pension income to the IRS. Starting next year, income-tax withholding will apply to pension and annuity payments, just as it does to wages and salaries. The amount of tax withheld from regular monthly payments will depend on your income and number of exemptions. On certain irregular payments, withholding is pegged at a flat 10 percent.

You can request that tax payments not be withheld (knowing, however, that improved reporting systems will probably make it easier for the IRS to find you if you don't pay). Watch your mail for information, from the institution that remits your pension, on how to opt out of tax withholdings. Exempt yourself if you expect to owe little or no tax, or if you always have enough money on hand to meet your tax bills. Otherwise, consider accepting tax withholding as a convenient way of paying taxes.

2. Tips are notorious for not being reported to the IRS. Starting next year, most restaurants with more than 10 employes might have to add an assumption about tips to the amount of income reported on employes' W-2 forms. Fast-food and carry-out places are exempted.

3. The gravy train is over for people who borrow money from their pension plans in order to buy a second home or finance other investments. As of Aug. 14, 1982, any loans from your plan will be treated as a taxable distribution, to the extent that they exceed $50,000 or one-half your vested interest in the plan, whichever is less. The new law says that you can, in any case, borrow up to $10,000. But as a practical matter this minimum will probably not apply to most people whose vested interest in the plan is less than $10,000, says Ted Rhodes of Miller and Chevalier in Washington, D.C.

Any part of a pension-plan loan that is not repaid within five years will also be treated as a taxable distribution, with one exception: You may take longer to repay money borrowed to build, buy or rehabilitate your principal residence or that of a qualified family member.

If you had a big loan outstanding on Aug. 14, you can continue repaying it on whatever schedule the loan originally allowed for. But you can't refinance it with a new loan that exceeds the $50,000 maximum. People with big loans maturing over the next year get a one-year grace period to make repayment.

This provision is going to squeeze a lot of people with loans outstanding. You might have to borrow money from an outside lender to get your plan repaid on time.

You will not be able to take any new loans from your pension plan, until your old loans have been brought below the allowable limits.

4. If you incorporated your personal-service business chiefly to get a better pension plan, the new law may have wiped your corporation out.

Starting next year, the IRS can--and will--challenge incorporated professionals, sports and TV stars, business consultants and technicians who work mainly for single organizations. If challenged, you could lose your medical plan deductions; the earnings of your company could be added to your personal income; and you might even have to fight for your latest pension plan contribution. People in this position should get advice about disincorporating.

5. Tax penalties are up sharply for people who buy gimmicky tax shelters of dubious economic value. If you're caught with a hokey art or book shelter, or an overvalued oil well, it's likely to cost you a lot of money. Be advised: Stay away from this stuff.

Other penalties are up, too. If you owe taxes and don't file a return, your penalty formerly couldn't exceed 25 percent of the taxes due. Starting next year there's a minimum penalty: $100 or the entire tax due, whichever is less. People owing small amounts of taxes will find it costlier not to file.

People who file frivolous returns will now be charged $500, in addition to any other penalties due. This covers tax protesters who, for example, claim they don't owe taxes because the United States is off the gold standard, or who hamper the IRS by leaving out key information on their returns or by adding up their taxes wildly wrong.

Some employers and other payers will also be required to file more or better information returns to help the IRS track down people who are not paying their legal share of the tax burden.