Rule 1:
Subject-verb agreement: the verbs are customarily followed by the subjects, and they must agree with the subjects according to their number and person. See the rules of Subject-Verb Agreement.
Rule 2:
The variability of the verbs mostly depends on different tenses of the sentences. A form of verbs depends on the time the actions have been performed. See the structures and details of The Present Tense, The Past Tense, and The Future Tense.
Rule 3:
The verbs are also related to the structures of different sentences and clauses. A clause has only one verb. In fact, a clause cannot contain more than one finite verb but can have participles (without auxiliaries), infinitives, and gerunds.
Example:
- I wanted (main verb) to go (infinitive) to the wedding.
- Swimming(gerund) is(verb) a good exercise to keep (infinitive) your body fit and healthy.
- Pray (verb) for the departed (past participle)
- Don’t get(verb) down from a running(present participle) bus.
Rule 4:
Clauses can be connected by conjunctions and connectors. Some connectors take some specific forms of verbs. Conditionals use the conjunction if and it has some different sentence structures. See conditionals.
Rule 5:
The connector since has two usages.
- If since is used to indicate a cause, the verbs of the two clauses will be of the same tense.
Example:
- I didn’t go since you didn’t come.
- Since you were busy, I didn’t disturb you.
- If since is used to indicate time, the verb of the second clause will be different from the first.
Example:
- We haven’t seen(present perfect) each other since he left(simple past).
- It has been 20 years since he played football.
- 20 years passed(simple past) since we had met(past perfect).
- I could not sleep well for a single night since you had left me.
Rule 6:
No sooner had, hardly had, scarcely had, etc. are the adverbials that require a past perfect tense and a past indefinite tense for the sentence.
Example:
- No sooner had I reached home than she left.
- Scarcely had the police reached when the man died.
- Hardly had I finished writing before the teacher ordered to stop writing.
Rule 7:
Coordinating conjunctions (and, but, or, yet, so) and although/though, as, because, till/until, when, whenever, as soon as, while, which, what, that, etc. connect two clauses which have the verbs of the same tense.
Example:
- We went to London when we were young.
- I got up, and he left the room.
- As soon as I came here, he greeted me.
- I could not go there because I was sick.
Rule 8:
As conjunctions after and before are used to connect two clauses which use past perfect tense and past simple. The past perfect tense always comes with a clause of simple past tense. See the past perfect tense for details and examples.
Rule 9:
The conjunction lest requires a modal should in the following clause regardless of the tense of the first clause.
Example:
- Run faster lest you should miss the bus.
- He will work hard lest he should get fired.
- He worked hard lest he should get fired.