DEAR BOB: A year ago I subleased my dry-cleaning shop. The new dry cleaner is now four months behind in his rent. The landlord threatens to sue me for the back rent plus rent for the balance of the four-year lease. What should I do? Morris T., Washington.

DEAR MORRIS: See your attorney. Your first mistake was to sublease. A better alternative is to get the land-lord's consent to assign the lease to the new tenant. That would relieve you of liability.

A sublease means you are still liable for the rent if the subtenant won't pay. Find out why the subtenant isn't paying the rent. Maybe you'll have to evict him and take back the store so you can pay the rent.

DEAR BOB: I phoned the IRS to apply for an extension of the 18-month time limit on the "residence replacement rule." The man said such extensions aren't granted. We can't afford to buy a house now. But we can't afford to pay our sale profit tax either. What should we do?Chuck W., Wheaton.

DEAR CHUCK: Sorry, but the "residence replacement rule" time limits are extended only if you join the armed forces. To defer your home sale profit tax, you must buy a more expensive replacement principal residence within 18 months before or after the sale. But you can take up to 24 months after the sale to build a new home. Your tax advisor has details.

DEAR BOB: We are buying a new condominium in Florida. The agent asked if we wanted title insurance. We never had it on our old home. The builder surely must have had a title search. Do we need title insurance? Thelma N., Annapolis.

DEAR THELMA: Yes, you especially need title insurance when buying newly constructed property because liens may have been recorded during the construction period. A title insurance policy protects against title claims as long as you own the property. The cost is very reasonable.

DEAR BOB: I will become 55 on Dec. 27. My house is listed for sale now. If I accept a purchase offer before I become 55, will I lose that big new $100,000 profit tax exemption? Jessie M., McLean.... I understand that the new tax law gives me a 10 percent tax credit if I remodel my store building. What is the effective date? John A.

DEAR JESSIE and JOHN: The 1978 tax bill is loaded with benefits for property owners. But it's tricky, so check the effective dates. Here's a quick summary, but see your tax advisor for full details.

The new "over 55 rule" tax exemption of up to $100,000 profits on the sale of your principal residence applies to title transfers after July 26, 1978. To qualify, the seller must be 55 or older on the title transfer date and have owned and lived in the principal residence at least three of the five years before sale. This big benefit can be used even if you used the old "over 65 rule" that applied to sales of principal residences before July 26.

The long-term capital gain tax reduction affects payments received after Oct. 31, 1978. This big tax reduction makes only 40 percent (the old law taxed 50 percent) of your long-term capital gains taxable.

DEAR BOB: If I rent a condominium I own to my mother for $100 per month (that's all she can afford), can I take a tax reduction for my loss? What about the depreciation? Gertrude R., Arlington.

DEAR GERTRUDE: If your rental is far below the condo's market value and is not profit-motivated, the IRS limits your tax deductions for that rental. Report the rental income in the usual way on Schedule E of your tax returns. But the expenses must be itemized deductions. If you don't itemize your tax deductions, you get no deductions for rental expenses.

Of course, your mortgage interest and property tax are always fully tax deductible. You can also deduct operating costs up to the amount the rent exceeds the interest and taxes, plus depreciation up to the total amount of rent income exceeding interest, tax and operating expenses. In other words, no tax loss deduction is allowed for non-profit rentals. Your tax advisor has more details.

DEAR BOB: If a homeowner keeps his home until he dies, does the person inheriting it owe any income tax? Do I understand the law correctly that if you keep selling and buying more expensive replacement homes, when you die, if your heirs sell the home, they owe no capitalgains tax?Harry S., Washington.

DEAR HARRY: The 1978 tax bill changed the answer to your question, in your favor.

The 1978 tax bill restored the pre-1977 tax rule that persons inheriting property take it at its value on the day of the decedent's death. This means the stepped-up basis, over the decedent's lower basis, escapes capital-gain tax if the inheritor sells the property. This temporary tax rule, called the "stepped-up basis rule," applies only until Dec. 31, 1979.

The Bruss report "How to Sell Your Home With or Without an Agent" is available for 25 cents plus a STAMPED self-addressed envelope sent to Robert J. Bruss, P.O. Box 6710, San Francisco, Calif. 94101.