In addition to the auxiliary verbs avoir and être, French has a number of semi-auxiliary verbs. Semi-auxiliary verbs are conjugated in order to express various nuances of time, mood, or aspect and are followed by an infinitive. Some semi-auxiliary verbs are equivalent to modal verbs in English and some are verbs of perception. Here are uses and meanings of some common French semi-auxiliary verbs:
Aller
(in present or imperfect tense) to be going to
Je vais étudier - I'm going to study
J'allais étudier - I was going to study
(any tense) to go to/and:
Va chercher les clés - Go and look for the keys
Je suis allé voir mon frère - I went to see my brother
(any tense) used to emphasize the verb that follows:
Je n'irai pas répondre à cela - I'm not going to dignify that with a response
Je vais te dire une chose - Let me tell you something
Lesson on aller
Devoir
(any tense except conditional and past conditional) indicates obligation or necessity
J'ai dû partir - I had to leave
Tu dois manger - You must eat
(conditional) should; (past conditional) should have
Je devrais partir - I should leave
Il aurait dû nous aider - He should have helped us
Lesson on devoir
Faillir indicates that something almost happened
Il a failli tomber - He almost fell
J'ai failli rater l'examen - I nearly failed the test
Lesson on faillir
Faire (causative construction) to make something happen, have something done, make someone do something
J'ai fait laver la voiture - I had the car washed
Il me fait étudier - He's making me study
Lesson on faire
Laisser to let something happen, let someone do something
Vas-tu me laisser sortir ? - Are you going to let me go out?
Laisse-moi le faire - Let me do it
Lesson on laisser
Manquer (followed by optional de) indicates that something was about to happen or nearly happened
J'ai manqué (de) mourir - I almost died
Elle a manqué (de) pleurer - She nearly cried
Lesson on manquer
Paraître to appear/seem to
Ça paraît être une erreur - That appears to be an error
Il paraissait être malade - He seemed to be sick
Lesson on paraître
Partir to leave in order to, to go to
Peux-tu partir acheter du pain ? - Could you go out and buy some bread?
Il est parti étudier en Italie - He went to study in Italy
Lesson on partir
Passer to call/drop in on, call for, to go to
Passe me chercher demain - Come pick me up tomorrow
Il va passer voir ses amis - He's going to drop in on his friends
Lesson on passer
Pouvoir can, may, might, to be able to
Je peux vous aider - I can help you
Il peut être prêt - He might be ready
Lesson on pouvoir
Savoir to know how to
Sais-tu nager ? - Do you know how to swim?
Je ne sais pas lire - I don't know how to read
Lesson on savoir
Sembler to seem/appear to
Cela semble indiquer que… - That seems to indicate that…
La machine semble fonctionner - The machine appears to be working
Lesson on sembler
Sortir de (informal) to have just done something
On sort de manger - We just ate
Il sortait de finir - He had just finished
Lesson on sortir
Venir
to come (in order) to
Je suis venu aider - I've come to help
venir à - to happen to
David est venu à arriver - David happened to arrive
venir de - to have just done something
Je viens de me lever - I just got up
Lesson on venir
Vouloir to want to
Je ne veux pas lire ça - I don't want to read that
Veux-tu sortir ce soir ? - Do you want to go out tonight?
Lesson on vouloir
Page 1 - French semi-auxiliary verbs
Page 2 - More semi-auxiliary verbs, word order, agreement

