After verbs of perception (see, watch, hear, listen, feel, smell, notice, observe etc.), the present participle expresses an action that continues over a period of time, regardless of whether we perceive the whole action or only part of it:
Since the simple gerund and the present participle have the same form (verb-ing), sometimes it can be difficult to decide whether an -ing form is a gerund or a present participle.
It may be worth remembering that a gerund always functions as a noun:
The present and past participles can be used as adjectives. When used in this way, they are sometimes called participial adjectives and the present participle has an active meaning, while the past participle usually has a passive meaning. They can occur:
