Tense

general rule: won't change the tense of the sentence without sufficient reason

1. present tense
2. past tense
3. future tense

4. -ing

5. perfect tense (important)
* present perfect
* just, already, yet, ever, never, for two years, since 1973, in the last decade, in the past ten years, recently
* past prefect
* by the time, by the end of 1984
6. main/sub sentence tense disagreement
* object sentence use present tense if it represents a fact or truth

7. be v.-ed
* by

8. virtual
* virtual present: were/v+ed (should/would/could/might + v)
* virtual past: had v+ed (should/would/could/might have + v+ed)
* virtual future: should/were to + v.ed (should/would/could/might + v)

9. subjective judgment
* advise, demand, require, insist, motion, recommend, suggest, mandate, order
* it is (essential, imperative, important, necessary, urgent, vital)
* were + sub. / should/had + sub. + v.