Is It Had Seen or Had Saw?

Saw is the PAST TENSE of the verb see, and usually comes immediately after NOUNS and PRONOUNS. Seen is the PAST PARTICIPLE of the VERB see. Generally, seen is used alongside have, has, had, was or were in a sentence to make COMPOUND VERBS.

Which is correct had seen or had saw?

Typically we use the past simple (saw) for events in the past and the past perfect (had seen) for events further in the past.

Have they seen or saw?

'Saw' is the past tense of the word 'see' while 'seen' is the past participle. Typically, 'saw' comes immediately after the noun or pronoun. For instance, “Steve saw the movie.” 'Seen' is never used as a standalone verb and is generally accompanied by words such as 'have', 'had', 'was', among others.