When we talk about things that are generally or always true, we can use:
- If he gets there before me, ask him to wait.
- When you fly budget airline, you have to pay for your drinks and snacks.
- Unless you need more space, a small car is big enough for one person.
Note that we are not talking about a specific event but something which is generally true.
In the condition clause, we can use a variety of present forms. In the result clause, there can only be the present simple or imperative.
- If you visit London, go on the London Eye.
- If unemployment is rising, people tend to stay in their present jobs.
- If you’ve done that, go and have a coffee.
- When you go on holiday, take plenty of sun cream. It’ll be very hot.
- When I’m concentrating, please don’t make so much noise.
- When I’ve finished an article, I always ask Kate to read it through.
Notice that ‘unless’ means the same as ‘if not’.
- Unless he asks you politely, refuse to do any more work on the project.
- Unless prices are rising, it’s not a good investment.
- Unless you’ve been there yourself, you don’t really understand how fantastic it is.