In Japanese, のは
behave in a very similar way to the words ‘that’ and ‘which’ in English. This means that they perform a task called nominalization
is when a phrase is treated in the same way as a single noun.
He is the one that eats a lot. (Literally ‘the one that eats a lot, he is’)
I don't like riding the bus. (Literally ‘that which is riding a bus, I hate’)
Rather than a single noun, we can see from these examples that the phrases 沢山（たくさん）食（た）べるのは
are behaving in the same way that a single noun would. In English, this can either mean ‘that which’, or ‘the one who/that’.
This construction can be used after verbs in any tense, except for the polite ます or ません, which are only used at the end of a sentence, or certain clauses.
He likes running.
My daughter is the one that ate all of the candy.
Imada-san is that one who is climbing the mountain.
の cannot be used as a substitute for こと in set expressions like ことができる, or ことがある. It also may not be followed by だ, である, or です, as this would become the explanatory ～んです, or のです in that case.