Fill suitable form of verb:
We were good friends. We ___________ (know) each other for a long time.
a. has known
b. had known
c. have known
d. is knowing

Hint: Consider the duration for which the action has been going on. Consider if it is still going on or if it has ended.

Complete answer:

You use the Simple Past Tense when the action is indefinite in the past. It doesn’t make use of any auxiliary verb.
You use the Past Continuous Tense when the action is continuous in the past. The infinitive form of such verbs is preceded by the auxiliary was.
You use the Past Perfect Tense when the action has been completed in the past itself. The infinitive form of such verbs is preceded by the auxiliary had.
You use the Past Perfect Continuous Tense when an action is expected to continue up until a point in the past. The infinitive form of such verbs is preceded by the auxiliary had been.
Let us examine the options given to us.
a. has known - This option is framed in the present tense. Therefore, it is incorrect.
b. had known - This option is framed in the past tense. Therefore, it is correct.
c. have known - This option is framed in the present tense. Therefore, it is incorrect.
d. is knowing - This option is framed in the present tense. Therefore, it is incorrect.

So, the correct answer is “Option b”.

Note: The verb ‘were’ indicates that the friendship was in the past. This means that the answer also has to be framed in the past tense.