Often, we use qui to form a question, but sometimes it can be part of a sentence too. When it refers to a person, it means who, otherwise it means that.
Je vois un homme qui marche seul.
I see a man who is walking alone.
J’ai une sœur qui habite en France.
I have a sister who lives in France.
Je prends le livre qui est sur la table.
I’m taking the book that’s on the table.
Some verbs are followed by à, like penser à, s’intéresser à, or by de, like rêver de. Unfortunately, there is no way to tell when you have to use de or à, so you just have to memorize them. C’est tragique !
Je pense à mes études.
I think about my studies.
Tu t’intéresses à la musique.
You are interested in music.
Nous rêvons d’acheter une maison.
We are dreaming of buying a house.