How do you form the conditional?

In order to form the conditional in French, you need to take the infinitive of the verb you want to use (e.g. manger - to eat) and add the ending depending on which pronoun you wish to use.

Je mangerAIS - I would eat

Tu mangerAIS - You would eat

Il / Elle mangerAIT - He/She would eat

Nous mangerions - We would eat

Vous mangeriez - You (plural or polite) would eat

Ils / Elles mangeraient - They would eat

However there are exceptions to the rule, with the irregular verbs you do not use their original infintive form however the endings remain the same.

Faire (to do) - Je ferAIS - I would do

Aller (to go) - J'irAIS - I would go

Etre (to be) - Je serAIS - I would be

Avoir (to have) - J'aurAIS - I would have

Pouvoir (to be able to) - Je pourrAIS - I would be able to 

Devoir (to have to) - Je devrAIS - I would have to 

Savoir (to know) - Je saurAIS - I would know

Vouloir (to want) - Je voudrAIS - I would like