It depends on the verb - does it end in an 'er', 're', or 'ir'? With regular verbs, you simply take the stem (the part without the aforementioned ending) and then for 'er' verbs, add 'e', 'es', 'e', 'ons', 'ez', or 'ent', for 're' verbs, add 's', 's', nothing, 'ons' , 'ez', or 'ent', and finally for 'ir' verbs, you add 'is', 'is', 'it', 'issons', 'issue', or 'issent'.