Most verbs of perception (e.g. hear, see, watch, notice) are followed by object + infinitive (without to).
- I heard him cry. (NOT I heard him to cry.)
- I watched them play. (NOT I watched them to play.)
- I saw her cross the road. (NOT I saw her to cross the road.)
Most of these verbs can also be followed by –ing forms. Note that there is usually a difference of meaning. We use infinitive forms after these verbs to say that we hear or see the whole of an action or event. On the other hand, -ing forms suggest that we see or hear an action in progress.
Compare:
- I once heard him give a talk on international politics. (I heard the whole talk.)
- As I walked past his cabin, I heard him talking with his secretary. (I didn’t hear the whole talk.)
An –ing form is not usually used to refer to a momentary action or event.
- I saw the bomb explode. (NOT I saw the bomb exploding. The action mentioned here (explosion) doesn’t last for more than a few moments.)
Sometimes an –ing form can suggest repetition.
- I saw the children throwing stones at the dogs. (= The children kept throwing stones.)
After can see and can hear, only the –ing form can be used.
- I could see him climbing up the tree. (NOT I could see him climb up the tree.)