Answer:
In English you just (normally) have to add 's' but sometimes, 'es' in some cases you have to change f's to v's (eg leaf, leaves)
in latin you have to decide which declension the noun is in, if its 1st, it ends in e (eg. feminae) if its 2nd it ends in i (eg. pueri) and 3rd it ends in 'es' (eg. mercatores or senes)