TO HEAR, TO LISTEN TO, TO SEE and TO WATCH are verbs of sensory perception.
These verbs are followed by a verb +ing to indicate a part of an action, not a complete one.
e.g. I listened to my sister playing the piano = (I didn’t stay there and listen the whole time she played).
I heard her singing . = (She was singin before and /or after I heard her.)
These verbs are followed by the infinitive (without to) to indicate a complete action.
e.g. I listened to my sister play the piano. = (Till the end of the performance.)
I heard her sing. = (From the beginning to the end.)
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