Qual è la differenza tra It e This ?Sentiti libero di rispondere con un esempio.

It is a pronoun that takes the place of any noun that's not a person.
This is a determiner that describes a noun close to the speaker.

This is a car. It is very cool.
The car is close to the speaker, and in the second sentence, "car" is replaced by the word "it" since it's understood that the speaker is talking about a car.

"It" can also be used when speaking about a general situation. This is called the "ambiguous it".

It is raining.
It is cold.
It's time to go home.

In all three sentences "it" isn't really replacing a noun, but it's taking place of the general situation.