Conditional clauses can begin with unless. Unless means something similar to ‘if … not’ or ‘except if’.
The verb forms in the examples are similar to sentences with if: we use the present simple in the unless-clause and shall, should, will, would, can, could, may or might in the main clause:
Unless I phone you, you can assume the train’s on time. (If I do not phone you /except if I phone you, you can assume the train is on time.)
We’ll have to cancel the show unless we sell more tickets at the last minute. (We’ll have to cancel the show if we do not sell more tickets/except if we sell more tickets at the last minute.)
We don’t use unless for impossible conditions:
If the government had not raised food prices, there would not have been so many protests.
Not: Unless the government had raised food prices …
We don’t use unless and if together:
We’ll go to the coast tomorrow unless it rains.
Not: We’ll go to the coast tomorrow unless if it rains.
Should you (Should with inversion)
In formal situations, we can use should + subject (s) + verb (v) instead of if:
Should you wish to cancel your order, please contact our customer service department on 02317 6658932. (or If you should wish to cancel your order …)
Should your child become anxious or nervous about any activity, it is a good idea to inform the team-leader. (or If your child should become …)
Had you (Had with inversion)
In formal situations, we can use had + subject + verb instead of if in third conditional sentences:
Had I known you were waiting outside, I would have invited you to come in. (If I had known you were waiting outside …)
Had Margaret realised she would be travelling alone, she would never have agreed to go.
If + were to
In formal situations, we can use if + were to when we talk about things that might happen but which we think are unlikely:
If the Prime Minister were to resign, there would have to be a general election within 30 days.
In even more formal styles, we use were + subject-verb inversion + to-infinitive:
[V]Were [S]we [to -INF]to give up the fight now, it would mean the end of democracy in our country. (If we gave up the fight now …)
[V]Were [S]the economy [to -INF]to slow down too quickly, there would be major problems. (If the economy slowed down too quickly …)
As long as, so long as, providing, etc.
Sometimes we need to impose specific conditions or set limits on a situation. In these cases, conditional clauses can begin with phrases such as as long as, so long as, only if, on condition that, providing (that), provided (that).
As long as is more common in speaking; so long as and on condition that are more formal and more common in writing:
[to a group of children]
You can play in the living room as long as you don’t make a mess.
So long as a tiger stands still, it is invisible in the jungle.
The bank lent the company 100,000 pounds on condition that they repaid the money within six months.
Providing (that) is more common in speaking; provided (that) is more formal and more common in written language:
[talking about rail travel in the UK]
You can get a senior citizen’s reduction providing you’ve got a railcard.
They may do whatever they like provided that it is within the law.
Or and otherwise
We often use or and otherwise with conditional meanings:
You’ve got to start studying, or you’ll fail all those exams. (If you don’t start studying, you will fail the exams.)
[talking about sending a package by mail]
We’d better send it express, otherwise it’ll take days. (If we do not send it express, it will take days.)
Supposing may be used with a conditional meaning. It can be used in first, second or third conditional sentences. The speaker invites the listener to imagine a situation:
Supposing I don’t arrive till after midnight, will the guest-house still be open? (Imagine if I don’t arrive till after midnight …)
Supposing you lost your passport, you’d have to go to the embassy, wouldn’t you?
Supposing he hadn’t recognised us – he might never have spoken to us.