What does this sentence mean?
I saw him wash the car
Does it mean "I saw him when he was washing the car"?
What does this sentence mean?
I saw him wash the car
Does it mean "I saw him when he was washing the car"?
It doesn't quite mean "I saw him when he was washing the car".
As I have explained elsewhere
Verbs of perception like see, hear, watch, feel take both -ing-form and bare infinitival clauses as complements, but there is a slight difference of aspect between them:
The infinitival complement implies that what is perceived is a completed action.
He watched me play means that he watched until I was finished playing.
The -ing-form complement implies that the action continues while it is perceived, but is not necessarily finished during that period.
He watched me playing means that he watched for some time while I played, but implicates that he stopped some time before I finished.
So "I saw him wash the car" means that I saw him complete his action of washing the car, right down to finishing the job. "I saw him when he was washing the car" is a (very rough) paraphrase of I saw him washing the car.
It could mean a couple different things, depending on emphasis and context. You've listed one possible meaning:
I saw him when he was washing the car.
By including when in this version, this particular sentence seems to emphasis time. In other words:
"When did you see him?"
"I saw him when he was washing the car."
Another possible meaning:
I saw that he washed the car.
This version emphasizes that you know he washed the car because you witnessed the event:
"The car looks dirty – are you sure he washed it?"
"Yes, I saw that he washed the car."
Or, perhaps we want to emphasize who washed the car:
I saw that it was him (and not someone else) who washed the car.
"Somebody washed my car – I wonder of it was Deborah."
"No I saw that it was him (and not Deborah) who washed the car.
When you take a simple sentence like yours, and ask what it means, it's hard to say precisely what it means without surrounding context. The gist of the original sentence is simple:
but any other meaning is conveyed more by the situation and the surrounding conversation than it is by the individual words themselves.
You're overcomplicating things. It simply means that he was seen as he was washing the car and it doesn't mean that the act was completed. It only takes a split second of observing this action to be able to make that statement