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't 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't be able to buy a new car unless he wins the lottery.
He won't 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’ll be back by the weekend, unless there's a train strike.
I’ll be back by the weekend, if the train drivers aren't on strike.

We’ll play tennis on the outdoor courts on Friday, unless it rains in which case we’ll play indoors.
We’ll play tennis on the outdoor courts on Friday, if it doesn't rain. If it does rain, we’ll 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…not 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's 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't phone me if you get into trouble!
Don't 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't 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't 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'm 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'm not sure if I’ll 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’ll be fully fit when the new football season kicks off, we don't yet know.
 
Compare the following conditional use with whether:
 
I’ll come with you to the hospital, if you want me to.
I'm 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