Rule 1:

The conjunction as if/as though takes simple past/past perfect tense in the following clause.

Subject + simple present + as if/ as though + subject + past simple

Subject + simple past + as if/ as though + subject + past perfect

Example:

• Robert talks as if he were the prince. (Were is the only ‘be verb’ in this kind of sentence)
• Latham played as though he had seen the ball very clearly.
• I slept as if I had been dead.
• He behaves as if he were her husband.

Rule 2:

The connector since has two usages.

1. 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.

2. 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 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 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.