If you haven't yet figured your '83 taxes, you should take note of some twists

If you haven't done your 1983 taxes yet, you may notice some changes in the form this time. Although there are fewer rules to contend with than last year, these rule changes can be confusing and easily overlooked, according to the New Hampshire Society of Certified Public Accountants.

Last year, those changes caused problems for taxpayers and the Internal Revenue Service. Two million married couples neglected to take a hefty new deduction for two-earner couples, according to the IRS, and 377,000 others made mistakes on the new child- and dependent-care formula. To avoid losing out on tax savings and to keep out of trouble with the IRS, taxpayers should take a closer look at what's new on the 1983 income tax return.

The deduction for married couples, when both spouses work, is twice what it was last year. A two-earner couple should subtract 10 percent (up to $3,000) from the earned income of the lower-paid spouse. Take this deduction in Part III of Form 1040A, or on Line 29 of Form 1040.

Medical expense deductions will be calculated against a stiff new formula this year. People should deduct only those medical and dental expenses that exceed 5 percent of their adjusted gross income. That means an individual with a more than $750.

Besides that, the separate deduction for half of a person's health insurance premiums (up to $150) has been eliminated. But the cost of medical insurance premiums with other medical and dental expenses may be deducted.

The casualty and theft loss deduction, which works like the medical decuction , also changes on 1983 tax returns. Now you may deduct only the unreimbursed casualty and theft losses that exceed 10 percent of income. You may also reduce each casualty loss by $100. The new casualty and theft loss rules severely limit the size of deductions. For instance, someone with a $15,000 adjusted gross income whose basement is flooded can deduct only unreimbursed losses above $1, 600.

Another change in the IRS procedure concerns the reporting of state and local income tax refunds. Refunds received from state and local government have always been taxable, but now, for the first time, local government agancies are sending the federal government statements on the amount of your refund. A copy of that statement, the 1099-G, will also be sent to you. You must add the amount of your 1982 refund to your 1983 taxable income, unless you did not deduct state or local income taxes last year.

Motorists who use their cars for business may now deduct 20.5 cents a mile for the first 15,000 miles of business travel. That's a half-cent increase in the standard mileage rate deduction over last year. Other mileage deductions are unchanged. It's 11 cents a mile for business travel over 15,000 miles and 9 cents a mile for medical and volunteer travel.

In addition to changes in rules, the IRS has altered its forms. Form 1040EZ, introduced last year to simplify filing for single people who earn less that $50 ,000, looks different. The new format allows for optical scanning of the EZs, which promises to speed processing of returns while lowering IRS labor costs.

Form 1040A, the traditional short form, has also undergone changes so that more people will be able to use it. You can now use the short form to adjust your income for a contribution to an individual retirement account (IRA) by using Line 11 of that form. Previously, taxpayers had to use Form 1040, the long form, to report an IRA contribution. In addition, you can now use Part 4 of the short form to take a credit for child- and dependent-care expenses. If you use Form 1040, you can still take the credit by attaching Form 2441.

Paying attention to the changes in filing your income tax return will save you and the IRS trouble and money, and it lessens the chance that you will be audited.

In 1982, more than 1.4 million tax returns were audited, and the IRS found an additional $3 billion in tax liabilities from individual taxpayers. Ironically, IRS audits also uncovered some $500 million in overpayments to the government in 1982, resulting in refunds on 114,602 returns.

Among the chief signs the IRS looks at in deciding who will be audited are excessive losses for tax shelters; very high medical deductions; unusual business and travel expenses; high casualty or theft loss deductions; excessive sales tax deductions; and steep alimony payments.