Tell me please what is the difference between the following sentences.
I saw him doing his homework.
I saw him to do his homework.
Not so long ago, I would have thought that the second sentence is incorrect, but I stummbled upon this sentence:
When a person is under stress, the chest may be seen to heave and contract rapidly.
If the author used the ing ending of heave without to there, would change the meaning somehow?
As others have said in the comment, the infinitival clause in your second example does not function as a complement; it functions as a purpose adjunct. It's pretty much similar to the complex preposition "in order to".
In this sentence:
When a person is under stress, the chest may be seen to heave and contract rapidly.
The "to" is obligatory because the main verb "see" is passive. Its complementation is therefore similar to the verb "make", which requires a to-infinival complement when passivized:
- We made it heave
- It was made to heave
