English.Web: Present Participle

To form a continuous tense:

I am learning English.

As an adjective:

There was a sleeping baby upstairs.

To replace a relative "who" or "that":

(A screen that explains a grammatical point=)
A screen explaining a grammatical point.

After verbs of sensation:

I saw him walking down the street.

You see the Infinitive sometimes with 'to' and sometimes without.
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