What a great question. I wish I had a great answer based on a grammatical rule. I would say that both sentences are correct, however one will hardly ever hear the first (the one without the verb "is"). Older Canadian English speakers grew up hearing old and current British English, as well as old and current American English. I know that I have read and heard "it" used as a direct object without the verb "be/is". I believe this use is an older British style and used by higher society. Here are some examples: I think it necessary .... I feel it unnecessary ... I feel it unimportant ... I find it helpful ... I find it redundent ... We deem it illegal ... The previous examples are not unusual to me, but I simply can't explain why the verb "is" can be left out of the clause. But, I would not hesitate to use these clauses, especially if I wanted to appear to be from the aristocracy (blueblood, noble, privileged class, highborn, imperial, kingly, queenly).

The second sentence is correct. "I think it is important to learn English". You can also use the contract form of "it is" ==> "it's". ;)

Both are correct!

The second , because a subject as 'it' should be before a verb , so you have yo add 'is' then it becomes 'it is' which is the verb to be in present ^^

Both are correct :) The second one: The grammar is correct. The first one: The grammar is not wrong, it is just a reduced sentence. Like this one: We need to have it to be done. --> We need to have it done.