I forgot versus I had forgot. What is the difference between the two phrases?
"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.
First off, "it's" Incorrect as "it" is. The latter sentences has not been phrased correctly. The latter sentence should have been "I had forgotten" if you like.
Stepping from there, we should be able to disseminate the difference quite naturally: The difference is essentially emphatic over the Nature of "what" happened, rather than "when." This "nature" is distinctively, Simple Past in the former -the expression of which is being narrated as the event (forgetting) happening "once"- and Simple Past Perfect in the latter -the expression of which is being narrated as the event (forgetting) happening "before a specific period in the past".
The original British English uses "forgot" and "got", not the US derivation "forgotten" and "gotten". Read Agatha Christie's Murder On The Orient Express to see how she uses "gotten" to give the American character her "voice".