"it has been three generations since" or "there has been three generations since"?

Question

Tell me please the difference between the following sentences.

It has been three generations since we became to use first computers.

There has been three generations since we became to use first computers.

Answer 1

Well, I think both sentences are incorrect mainly due to this phrase: "since we became to use first computers."

There are a number of ways you could say this. Here are some examples (assuming I correctly understand what you mean):

"... since the first computer was invented"

"... since we first used computers"

"... since we used the first computers" {you will need to define "first"}

Note that just saying "first" may be ambiguous with regards to the classification of the computers (i.e., are you talking about the mechanical, programmable, or digital computer?)

"... since we became the first [generation] to use computers" {Not sure if this is something we usually say}

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Getting back to your main question "It has been" vs "There has been", your first sentence is correct.

"It has been three generations since we ..."

Here, "has been" works fine because "three generations" is a single time period and is therefore considered singular. "It has been X years..." is common usage when we think of those years as a single time period.

Your other sentence

"There has been three generations since we ..."

is not common usage {Note that I don't say its incorrect. I am not confident about that claim}. It should be

"There have been three generations since we ..."

Because we are using "there" and not "it", we need to use "have" with the plural "three generations". It is easy to explain/understand if we change the sentence a bit:

"Three generations have passed since we..." and not "Three generations has passed since we..."

I personally don't prefer writing "There have been three generations since we..." in this exact form/construction. I would always go with "It has been three generations since we ...". But that is a matter of style and choice, I think.

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This should be an interesting read: An arduous ten years have passed.