I forgot versus I had forgot. What is the difference between the two phrases?
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"I forgot" is the simple past, expressing an action which took place once.
"I had forgotten" is is the simple past perfect, used to express an action taking place before a certain time in the past. This tense emphasizes what happened, not the duration thereof.
"I had forgot" is generally considered bad grammar, at least in my part of the US, because the correct past participle of "forgot" is "forgotten".
See http://www.ego4u.com/en/cram-up/grammar/tenses for examples of all English tenses.