One of the most common pitfalls for non-native English speakers occurs when people want to talk about a conditional. This can be a hard habit to break since it is a direct translation from Dutch.
The big blunder in the Netherlands is putting the will/would in the “if” part of the sentence:
Right: If I get a promotion, we will go on vacation.
Wrong: If I will get a promotion, we will go on vacation.
Right: If you ran a marathon, you would be really tired.
Wrong: If you would run a marathon, you would be really tired.
Sometimes, students are confused because they think that because we use the past tense (if you ran) then we are talking about something that happened in the past. Not true, this is just an idiosyncrasy of English: we use the past simple to talk about a future condition.
Another common problem is when people mix up if and when as if they are interchangeable–they’re not!
As a rule of thumb, conditionals always start with if:
If means that you are not sure something will happen. When means that you know it will happen, you’re just not sure if it will be tomorrow, next week, or next year.
Be careful: If you start a conditional sentence with when, it can change your meaning entirely and you may end up saying something you don’t want to.
If we rob this bank, we’ll be rich. (I haven’t decided yet if we will rob the bank. No need to call the police.)
When we rob this bank, we’ll be rich. (Decision made! We are so robbing the bank. Don’t say this too loudly.)
- Conditional Clauses/ Conditional Sentences (expertscolumn.com)