depuis, pendant et pour

There are three prepositions in French, depuis, pendant and pour, that are translated as 'for' and are used to indicate the duration of an event. 

depuis

If an event began in the past and is continuing into the present, use 'depuis' with the present tense. For example:

pendant

Pendant literally means 'during', but it is usually best expressed in English as 'for' plus a certain amount of time. It can be used with all tenses.

Pendant is used with the passé composé to indicate how long a completed activity lasted. Be careful not to use depuis with the passé composé in this context.

pour

The preposition pour is used  in a variety of contexts. When translated as 'for' in English, it may designate the duration  of  a future event with the verbs partir  and aller. Note that  the verb may be conjugated in either the present or future tense.