The basic rule is to use tenses that reflect the time order of events.
PAST
If an event happens before the present, it is put into the simple past; if it happens before the simple past, it is put into the past perfect, or past past.
| Present |
Today, Sharon runs to the store. |
| Simple past |
Yesterday, Sharon ran to the store. |
| Past past |
The day before yesterday, Sharon had run to the store, but she ran there again yesterday. |
Note that the past past (or past perfect) uses a helping verb, had, plus a past participle, run.
FUTURE
If an event happens after the present, it is put into the future.
| Present |
Today, Sharon runs to the store. |
| Future |
Tomorrow, Sharon will run to the store |
Note that the future uses a helping verb, will, plus a present participle, run.
