To have eaten

JumanahHe was seen to have entered means "he was seen in the roomIs it correct?

Not exactly. We cannot see him in the room; we just saw him go into the doorway.

JumanahHe was seen to have eaten the apple means he was seen after he ate the apple.

No, we saw him eat the apple; we did not necessarily see him afterward.

What's the difference between them?
He was seen to have entered the room.
He was seen to enter the room.

JumanahWhat's the difference between them?He was seen to have entered the room.He was seen to enter the room.

There is no real difference in meaning that I see.

He was seen to eat his breakfast.
He was seen eating his breakfast.
Are they different?

JumanahAre they different?

Not really. The first is the fact; the second is the process.

JumanahHe was seen to have entered the room.

He entered the room. The entering was seen as a completed action.

JumanahHe was seen to enter the room

He was only seen entering the room. The entering was not seen as a completed action.

Mister MicawberThe first is the fact

I have no issue with " He was seen eating his breakfast".
Just for the sake of certainty, am I correct that you agreed: "He was seen to eat his breakfast" is good English?

AnonymousJust for the sake of certainty, am I correct that you agreed: "He was seen to eat his breakfast" is good English?

I would agree that it is.

Even though English is not my native tongue, I have been well educated in English i nthe US and have practiced it religiously on a personal and professional level for 30 years. I am sorry I raised this question, but I honestly have never heard anyone uttered in that grammatical form, which I agree is not wrong. That said, it sounds weird to me. Thank for your comment BJ.3d97

AnonymousI honestly have never heard anyone uttered in that grammatical form

That's because it is a rather considered form; you would be much more likely to meet it written than spoken. (As is also the case with the other sentence, actually.)