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:
Si on gagnait au loto, on achèterait une maison.If we won the lottery, we would buy a house.
Si tu venais avec moi, tu ne le regretterais pas.If you came with me, you wouldn't regret it.
Je ne saurais pas quoi faire si j'étais toi.I wouldn't know what to do if I were you.
Si j'étais riche, j'achèterais un appartement à Paris.If I were rich, I would buy an apartment in Paris.
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.
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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