3.209 USAGE NOTE

'Hear', 'observe', and 'see' are not used with a 'to'-infinitive when they are active but they can be used with either a present participle or a 'to'-infinitive when they are passive.

You use them with a present participle when you want to show that the action described by the second verb took place over a period of time.

If a 'to'-infinitive is used, you are implying that the action was completed.

See also paragraph 3.206.