Conditional sentences.
A conditional sentence usually has two clauses, a main clause and a subordinate conditional clause. We use conditional sentences to imply that the events of the main clause depend upon those of the conditional clause. So:
conditional clause main clause
If I can produce the letter … the matter will be forgotten.
The speaker means that his employers will forget that he has lost a certain letter if (only if) he can show them it. But if he is unable to produce it, his carelessness will not be forgotten.
§ 20.
UNLESS
Most conditional clauses start with if, they are often called if-clauses. Unless is the negative form of if and means only if not:
If I can’t produce it … ~ Unless I can produce it …
We often use unless in threats (a) and warnings (b).
Unless you stop making that noise, I’ll scream!
You’ll be hungry later unless you eat now.
Not all negative if sentences can be transformed into unless sentences.
If he wasn’t told by Jane, he couldn’t have known. ~ Unless he was told by Jane, he couldn’t have known. (can be changed )
If Mr. Smith doesn’t come back, I’ll phone you. (cannot be changed )
Other possible subordinators include: as / so long as, provided / providing that (= but only if), on condition that, even if / even though. If one situation depends on another, if can be replaced by as / so long as, provided / providing that. To introduce questions suppose / supposing (that) … ? ( ~ What if … ?) can be used.
You can borrow my camera as long as you are careful with it.
I’ll go to the party provided you go too.
Suppose he himself gave her the injections …
§ 21.
IN CASE
In case appears similar to if and is often confused with it. But the two are completely different. An in case clause gives a reason for the action in the main clause.
Compare:
BILL: I’ll come tomorrow in case Ann wants me.
TOM: I’ll come tomorrow if Ann wants me.
In (a) perhaps Ann will want Bill, perhaps she won’t. But Bill will come anyway. His action doesn’t depend on Ann’s. In (b), a conditional sentence, Tom will only come if Ann asks him. His action depends on hers.
18. Choose the correct word or expression.
1. We’ll have a picnic tomorrow unless / provided it rains.
2. I’ll buy the car unless / as long as it’s not too expensive.
3. I’ll draw a map for you in case / if you can’t find our house.
4. He won’t forgive you unless / as long as you say you’re sorry.
5. Unless / provided you tell the truth, everything will be all right.
6. John might phone tonight. I don’t want to go out in case / if he phones.
7. Unless / providing you lend me the money, I won’t be able to undergo this operation.
8. This letter is for Ann. Can you give it to her in case / if you see her?
9. You can use my car in case / as long as you drive carefully.
10. We’ll be late unless / as long as we hurry.
11. You won’t find any accommodation unless / providing you book it in advance.
12. Pack a jumper to wear after dark if / in case the evening is cold.
§ 22.