Learn French: S'ils quittaient le système solaire, ils exploreraient une nouvelle galaxie. - If they left the solar system, they would explore a new galaxy.

The word si (if) drops its i and uses an apostrophe only when it is followed by il or ils. This is a strict rule in French to avoid two "ee" sounds in a row. Note that this doesn't happen with elle or elles—for those, you would keep the full word: si elles.

Why is quittaient in the imperfect tense (l'imparfait) if this sentence is about the future or a hypothetical situation?

This is the classic rule for the Second Conditional in French. To express an unlikely or hypothetical situation ("If A happened, B would happen"), the verb directly following si (if) must be in the imperfect tense. Even though English uses the simple past ("if they left"), French requires the imparfait here.

How is the conditional verb exploreraient formed here?

To form the present conditional for regular -er verbs, you take the entire infinitive of the verb (explorer) and add the imparfait endings. For ils, the ending is -aient. Put them together, and you get exploreraient (they would explore).

Why does nouvelle come before the noun it describes, but solaire comes after?