Simple Present: expresses events or situations that exist always, usually or habitually; they exist now, they existed in the past, and they probably will exist in the future. (Examples: 1 and 2)
Simple Past: At a specific time in the past the action happened. The action began and ended in the past.
(Examples: 3 and 4)
Simple Future: At a specific time in the future the action will happen, but it has not started yet. (Examples: 5 and 6)
1. It rains in Seattle.
2. I exercise everyday.
3. It rained yesterday.
4. It rained non-stop last month.
5. It will snow tomorrow.
6. The forecast predicts snow next week.