In this document you will find two types of big picture explanations of prepositions that express time. The first page is a chart that shows students some of the different prepositions that express time. Included in the chart: à, en, dans, depuis, pendant, pour. On the second page, you will find a brief explanation of il y a, pendant, depuis and pour. All have examples.
I'm confident that your students will benefit from other 'BIG picture' resources I've created; I've bundled 15 BIG picture top