Choose the correct alternative to complete the given sentence.
Alex didn’t come to see the film last night because he ______ it before.
a. saw
b. had seen
c. has seen
d. was seen

Hint:The student should read the sentence carefully to understand when the event is supposed to be taking place. Then the options should be carefully distinguished to find out whether they are correct or not.

Complete answer: A verb is a content word that denotes an action, occurrence, or state of existence. It is the word class that serves as the predicate of a sentence.
In the given question, we have to find out which of the given options includes the answer in the past perfect tense, as this is the perfective tense used to express action completed in the past.
Let us thus analyze the options given to us in this question -
Option (a.), 'saw', refers to ‘simple past tense’.
Therefore, option (a.) is incorrect as we need the answer in the past perfect tense, as this is the perfective tense used to express action completed in the past.
Option (b.), ‘had seen', refers to ‘past perfect tense’.
Therefore, option (b.) is correct as we need the answer in the past perfect tense, as this is the perfective tense used to express action completed in the past.
Option (c.), ‘has seen', refers to ‘present perfect tense’.
Therefore, option (c.) is incorrect as we need the answer in the past perfect tense, as this is the perfective tense used to express action completed in the past.
Option (d.), ‘was seen', refers to ‘simple past tense’.
Therefore, option (d.) is incorrect as we need the answer in the past perfect tense, as this is the perfective tense used to express action completed in the past.
Therefore, option b is correct.

Note: In this question, it will be helpful for the student to recall the concept of tenses. Tense is a grammatical category of verbs used to express distinctions of time. There are three tenses - past, present, and future.