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Alternatives to 'if'
Eveline Real from Brazil writes:
  Can you help me with the correct use of unless? Is there any mistake in these sentences: Unless I finish the report, I cannot take a rest. And - Unless he wins a lot of money, the poor man can鈥檛 buy a car.
 
 
Roger Woodham replies:
 
 
Unless
 
 We sometimes use unless instead of if... not in the sense of except if, especially if we are talking about present circumstances and conditions. The second sentence you have quoted, Eveline, is a very good example of unless used correctly:
 
The poor man won鈥檛 be able to buy a new car unless he wins the lottery.
He won鈥檛 be able to pay all the tax he owes unless he robs a bank.
 
Unless is well used here because it highlights an exception to what is generally true. It works very well in the following examples too when the focus is on exceptions to the general rule. Compare the following:
 
I鈥檒l be back by the weekend, unless there鈥檚 a train strike.
I鈥檒l be back by the weekend, if the train drivers aren鈥檛 on strike.

We鈥檒l play tennis on the outdoor courts on Friday, unless it rains in which case we鈥檒l play indoors.
We鈥檒l play tennis on the outdoor courts on Friday, if it doesn鈥檛 rain. If it does rain, we鈥檒l play indoors.
 
If we use unless in the above examples, we think it unlikely that there will be a train strike and unlikely that it will rain. Using if鈥ot suggests that there may be a rail strike or that it may rain.

In your other example sentence, Eveline, it is likely that you will finish the report at some stage, so until would sound more natural in this example:
 
I cannot take a break until I finish this report.
 
If you say:
 
I cannot take a break unless I finish this report.
 
grammatically it鈥檚 fine, but it sounds a bit strange, as if there is someone standing over you and forcing you to work in impossible conditions.

Compare the following and note the contrastive differences in meaning between if and unless:
 
Don鈥檛 phone me if you get into trouble!
Don鈥檛 phone me unless you get into trouble.
 
In the first, I am saying that I want nothing more to do with you, that I am disowning you. Don鈥檛 phone me under any circumstances. In the second, I am saying that you can phone me if you want to, but only if you get into trouble.
 
 
As long as / provided / on condition that / only if
 
 We can use these alternatives to if if we want to emphasize the conditions surrounding the action, i.e. one thing will happen only if another thing happens. We can also use so long as and providing (that) as alternatives to as long as and provided (that). On condition that is formally very explicit. Provided / providing are more formal than as long as / so long as. Compare the following:
 
We will lend you the money on condition that it is repaid within 12 months.
We will lend you the money provided (that) you can repay it within 12 months.
I don鈥檛 mind talking to the press, but only if my identity is protected.
You can have the day off today providing you agree to work a double shift tomorrow.
You can borrow my DVD player as long as you return it on Monday.
 
 
Whether
 
 In indirect speech, we can use if or whether to introduce reported yes/no questions:
 
Can you feed the cat for me while I鈥檓 away? - She wanted to know whether I could feed the cat for her while she was away.

Do you have any free time on Sunday? - I鈥檓 not sure if I鈥檒l have any free time on Sunday.
 
When there is a choice of two possibilities, particularly in a two-part question with or, we normally use whether:
 
Let me know whether you can come or not.
I asked him whether he wanted to stay in a hotel or a B&B.
Whether (or not) he鈥檒l be fully fit when the new football season kicks off, we don鈥檛 yet know.
 
Compare the following conditional use with whether:
 
I鈥檒l come with you to the hospital, if you want me to.
I鈥檓 coming with you to the hospital, whether you want me to or not.
 
In the first, where your friend is talking, there are a number of possibilities: you can go alone, go with a friend, go with your mother, etc, so if is used. In the second, where your mother is talking, there are only two possibilities: you either want her with you or not, so whether is used.
 
 
    
A speeding car
 
 
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