When (in French) is the verb NOT conjugated?

Sometimes, the verb "acheter" or "parler" stays the same. WHY? My teacher told my cl once, but I completely forgot! Sometimes, a sentence might go like: "Il achete un poire" or "Je parle francais." But then sometimes it goes "Olivier ______ acheter __________." When does it become that? Please help. If I don't get this down, I'm going to totally fail my grammar. T.T