They didn't go out last night. I would’ve seen <it> if they had <gone out>.

[Michaelll]

Is it natural to shorten the first sentence into the second one?

1. They didn't go out last night. I would’ve seen it if they had gone out.
2. They didn't go out last night. I would’ve seen if they had.

 

[emsr2d2]

What do you mean by "seen"? Are you saying you would have personally witnessed them leaving the house?

 

[jutfrank]

Yes, it's okay. You can use an if/whether-clause as a verb complement.

Did you see if they left?

You can think of if they left as the object of see—a kind of thing, in the sense of a fact about the world.

 

[Michaelll]

What do you mean by "seen"? Are you saying you would have personally witnessed them leaving the house?

Yes, I would have seen them go out.

• They didn't go out last night. I would’ve seen it (= them go out) if they had gone out.

 

[emsr2d2]

They didn't go out last night. I would have seen them go.

 

[Michaelll]

They didn't go out last night. I would have seen them go.

OK, thanks. What about these? Are any of B's replies natural here?

A: Did they go out last night?
B: No, I would’ve seen it if they had gone out.
B: No, I would’ve seen (it) if they had.
B: No, they didn't. I would’ve seen it if they had gone out.
B: No, they didn't. I would’ve seen (it) if they had.

 

[emsr2d2]

They're all possible but none of them are as natural as "I would have seen them" or "I would have seen them go".