1 I saw a reading man in the room.
2 I saw a man reading in the room.
How do they differ? Does the first one mean that the person seen is always reading and the second means that the man was reading only then?
1 I saw a reading man in the room.
2 I saw a man reading in the room.
How do they differ? Does the first one mean that the person seen is always reading and the second means that the man was reading only then?
When you put a verb before the noun it acts as an adjective.
There may be some contexts where it doesn't matter too much, for example "I saw a car speeding" is pretty much interchangeable with "I saw a speeding car". However, 'a reading man' just doesn't sound idiomatic to a native speaker. What is a reading man? A man who always reads? A man who is well-read? It doesn't have an idiomatic meaning.
This describes an action in the moment it happens. It communicates that you saw a [noun], and that you saw it doing a specific thing.
I heard a dog barking I saw a man running from police I noticed three cars racing down the street
The meaning here can be different, if a different interpretation exists. The adjective describes a frequent activity for the noun. However, it doesn't necessarily mean that the activity is being engaged in right now.
In other words, it communicates that you saw a specific type of [noun], but you did not specify what the [noun] was doing.
I saw a racing car on the beach
A "racing car" is a specific kind of car. It does not have to be racing all the time. It could've been parked on the beach.
He is in the reading room
The reading room is a room intended for reading, but that doesn't mean that "he" is currently reading right now.
If there is no different interpretation for [adjective] [noun], then the same meaning from [noun] [adjective] can be inferred.
I saw a speeding car
I saw a car speeding
A "speeding car" isn't a known type of car in the same sense that a "racing car" is. Therefore, it means the same as the [noun] [adjective] variant
I saw a barking dog
I saw a dog barking
Hypothesis: there are two different types of dog. Some breeds are able to bark. Other breeds do not. With this hypothesis, a "barking dog" would be its own thing (like a "racing car"). It would refer to a dog of a breed that is able to bark.
This isn't reality, though. That concept does not exist. Therefore, the two sentences mean the same thing.
I saw a hunting dog
I saw a dog hunting
However, change the example from "barking" to "hunting", and suddenly the outcome changes too.
A "hunting dog" can be interpreted as a dog breed that is commonly used for hunting (e.g. beagle, retriever, ...), and now the two sentences no longer mean the same thing.
The first sentence communicates that you saw a specific type of dog, but you did not specify what it was doing.
The second sentence communicates that you saw an unspecified dog, and that you saw it doing a specific thing.
Sometimes, when [adjective] [noun] does not have a different meaning, that doesn't always mean that it sounds idiomatically equivalent to [noun] [adjective].
I saw a reading man in the room.
This sounds grating to my ears. I'm not able to pinpoint exactly why this is okay for "speeding car" and not "reading man", but it is. For "reading man" is seems to very much imply that it is a specific type of man, but this interpretation doesn't exist, and therefore it is confusing.