When the condition is not met
Here the condition is not met, or is very unlikely: I don't have time, I'm not rich, I'm not you. We think it through anyway.
This is the second conditional, and English builds it the same way French does: a past-looking verb in the if-half and would in the other half. French uses two tenses you already know: the imparfait after si and the conditionnel présent in the main clause.
Three sentences to start with
Si j'avais le temps, je viendrais avec toi.
If I had time, I'd come with you.
Si j'étais riche, j'achèterais une maison en Provence.
If I were rich, I'd buy a house in Provence.
Si on avait une voiture, on pourrait partir ce week-end.
If we had a car, we could go away this weekend.
Possible or unlikely?
The same idea, two builds. It depends on how you judge the condition: if it can happen, si takes the present. If it is not met or is unlikely, si takes the imparfait.
The same condition, two judgements
Possible: si + présent
S'il fait beau, on ira à la plage.
If the weather is nice, we'll go to the beach.
Unlikely: si + imparfait
S'il faisait beau, on irait à la plage.
If the weather were nice, we'd go to the beach.
Both verbs change together: fait becomes faisait, ira becomes irait.
What you actually use it for
Three uses come up again and again in everyday French.
Wishes and daydreams
Anything you'd like to be otherwise – the classic lottery question.
Si je gagnais au loto, j'arrêterais de travailler.
If I won the lottery, I'd stop working.
Si on vivait à la campagne, on serait plus tranquilles.
If we lived in the country, we'd have more peace.
Si j'habitais plus près, je te verrais plus souvent.
If I lived closer, I'd see you more often.
Giving advice
The polite way to tell someone what to do. Learn si j'étais toi as a fixed phrase. Just as common is à ta place with the conditionnel alone.
Si j'étais toi, je refuserais.
If I were you, I'd say no.
À ta place, je prendrais le train.
In your shoes, I'd take the train.
Suggestions with half a sentence
A si-clause with no main clause and a question mark is a suggestion in French: how about …? You will hear it every day.
Si on allait au cinéma ?
How about going to the cinema?
Et si on prenait un café ?
What if we grabbed a coffee?
How this lines up with English
The mapping is unusually clean: the English past form in the if-half is the French imparfait, and English would is the French conditionnel. French has no auxiliary, though: would come is a single word, viendrais.
English if I were is a genuine subjunctive, the last one standing. French needs nothing of the kind: si j'étais is a plain imparfait.
