[Verb (plain form)]の

We have already learned that の can act as a noun when modified by an adjective:

赤いのを ください。
Please give me the red one.

Not surprisingly, の can also be used with verbs.

We have already seen that こと can be used to turn a verb into a noun:

テニスをすることが 好きです。
I like playing tennis.

の behaves in much the same way:

テニスをするのが 好きです。
I like playing tennis.

However there is a difference: こと refers to a verb in an abstract kind of way, “the act of playing tennis” or “to play tennis”, whereas の has a much more immediate feel to it “that I play tennis” or “when I play tennis”.