What's the difference? 나는 문을 닫는다 /나는 문을 닫다
나는 문을 닫는다 is said to be present tense
나는 문을 닫다

나는 친구를 만난다
나는 친구를 만나다
what is exactly the difference and what would change the meaning of it?

Question

What's the difference? 나는 문을 닫는다 /나는 문을 닫다
나는 문을 닫는다 is said to be present tense
나는 문을 닫다

나는 친구를 만난다
나는 친구를 만나다
what is exactly the difference and what would change the meaning of it?

Answer

닫다 and 만나다 are dictionary forms, not present tense.

Don't confuse active verbs and adjectival verbs, they're not the same.
Active verb: dictionary form 하다 → present declarative 한다 (+는/ㄴ다)
Adjectival verb: dictionary form 예쁘다 → present declarative 예쁘다 (exactly the same)

Since 닫다 and 만나다 are active verbs, the forms you're looking for are 닫는다 and 만난다.
Using 닫다 and 만나다 in a sentence is wrong like saying "I to close the door" in English.

I can't give you extra info about the usage of this form, because I'm not familiar with it.
I'm only giving you the grammatical info, hope it still helps you!