"There comes a time when" vs. "It comes a time when"

Question

What are the differences between the two sentences below?

1. There comes a time when you have to fight for what you want.
2. It comes a time when you have to fight for what you want.

I usually use 'There comes a time when', but I often see 'It comes a time when' on the internet.

Answer 1

Only (1) is correct.

The pronoun "it" can only anticipate real, logical subjects (appearing after the verb) when these are infinitival, gerundial, or nominal clauses:

• It's good to fight for what you want.
• It's not worth fighting for what you want.
• It's good that you have decided to fight for what you want.

Only the pronoun "there" can anticipate indefinite nouns or noun phrases:

• There comes a time when... (Here, "there" is the grammatical subject of the sentence.)

As an adverb of place, "there" can also be placed at the beginning of a sentence for emphasis. However, in this case "there" is NOT the grammatical subject. The grammatical subject only appears at the end as a result of a grammatical process known as "full inversion." Also notice that, in this case, the noun or noun phrase need not be indefinite:

• There comes my train at last!
• There comes Mary.

Answer 2

The short answer: Like Gustavson says, only use (1).

The long answer: It is only (1) in standard English. However, they are interchangeable in various dialects. The difference between "There is/are" and "It is" is a dialectal. In non-standard English of the rural American South, one is typically going to hear "It is", as in "It's a storm coming through" for "There's a storm coming." "It's" replacing "There is/are" occurs in other English forms, to the extent that it is a grammatical rule in African-American English. The varieties are why you have come across it so much.

However, when using standard English, "it's" :) only one option that you can choose. The first one.