Here's a hypothetical situation in English:
If we won the lottery, we would buy a house.
-> Here you can see that if is followed by a verb in the Simple Past (won), while the second clause uses the Conditional Present (would buy).
Now look at the French:
Si on gagnait au loto, on achèterait une maison.
-> Here you see that in French, si is followed by a verb in L'Imparfait (gagnait), while the second clause uses Le Conditionnel Présent (achèterait).
Here are more examples:
Note that to express hypotheses in French, you'll use L'Imparfait after si, and Le Conditionnel Présent in the second part of the sentence.
ATTENTION:
You can never use Le Passé Composé nor other past tenses in this context.
See other uses of L'Imparfait:
Expressing continuing action in L'Imparfait (imperfect tense)
Expressing habits or repeated actions in L'Imparfait (imperfect tense)
And to see how to conjugate in L'Imparfait:
Conjugate regular verbs in L'Imparfait (imperfect tense) and Conjugate être in L'Imparfait (imperfect tense)
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