These 2 expressions mean exactly the same thing (Possession and Obligation - I've got to go/I have to go) but there are important differences in grammar and register.
"I've got an idea" is less formal than "I have an idea", so it is more common in spoken English, and great for Informal Letters (General Training).
"I've got" tends to be used more often in the UK, whereas "I have" is more common in the US.
Compare "I haven't got" [NOT "I haven't"*] with "I don't have" (this causes problems).
4. What are the PAST, PERFECT and FUTURE forms?
This is more complicated. Check out my YouTube video below where I go through them. Leave your questions on YouTube or in the Feedback below.