This clear and concise grammar sheet explains how to use the expressions DEPUIS, ÇA FAIT, IL Y A and DEPUIS, ÇA FAISAIT, IL Y AVAIT in order to say SINCE or FOR. I use this in French 4 with my intermediate French students.
The concept is explained clearly FIRST in the present tense with examples 9 practice translation sentences (where students are first asked to highlight SINCE or FOR in the sentence and CHOOSE which expression they will use). Next the concept is explained once more in the imparfait tense with examples 9 practice translation sentences. Finally, there is a third practice exercise of 15 fill in the blank sentences where students will have to use context to decide whether to conjugate the verbs in the present tense or in the imparfait.