When should I use "qui" and when should I use "que"?

A relative pronoun, such as “qui” or “que”, is a word that is used to connect two parts of a sentence together when referring to someone or something that has already been mentioned earlier in the sentence. When deciding which relative pronoun out of “qui” or “que” should be used, we need to ask ourselves just one question: Is the noun that is being discussed in the relative clause (i.e. directly after “qui” or “que”) the subject, or the object of the action that is taking place within it?

If it is the subject of the action being discussed within the relative clause, then we use “qui” - for example, "Je n'aime pas le garçon, qui a mangé mon gâteau." Here, it is the boy who is performing the action within the relative clause as he himself is 'eating the cake'. Therefore, we use “qui” to denote that he is the subject of the action within the clause. However, if the noun is the object of the action being discussed within the relative clause, we use "que" - for example, "Oú est le gâteau que tu as acheté pour ma fête." Here, as the cake has been bought by someone else, it is the object of the action that is taking place within the relative clause - so,"que" is the relative pronoun that we use.