Let me know the difference between some english sentences for foreign students.

[Humble]

Hi,
Only N1 is correct - it's the construction known as complex object: I saw her dancing etc.
Regards

[Humble]

Oh, in this case my example is not the best.
We heard her unlock the door would be better.

[Jay G.]

Hello Humble,

Why are another sentences incorrect?
Please explain why.

Thank you

[mykwyner]

Sentence 1 is correct for the reason you stated (bare infinitive and/or object complement).

Sentence 2 is incorrect unless the object becomes a relative clause:

"...you will observe that something appears suddenly..."

Sentence 3 is correct because the relative clause is in the future tense.

I cannot imagine that any native English speaker will notice any difference in meaning among the three sentences.

[Jay G.]

Thank you so much, mykwyner.

So can I omit "that" in your sentence, just like the sentence 2?

Originally Posted by mykwyner

Sentence 2 is incorrect unless the object becomes a relative clause:
"...you will observe that something appears suddenly..."

And why is "...you will observe something that appears suddenly..." incorrect? Because of tense?