N5
の
Verb nominalizer, The one who..., That which...
沢山食べるのは彼です。
He is the one that eats a lot.
Explanation
「の」 turns an action into a noun, so that it can take particles like 「は」, 「が」 and 「を」: 「泳ぐのが好きです」 (I like swimming), 「走っているのは彼です」 (The one running is him). It goes after the plain form of a verb.
Mixed up with
With things you directly sense, such as seeing and hearing, only 「の」 works: 「子供が遊んでいるのを見る」.
Verbs of waiting and stopping also take 「の」.
Verbs of telling and conveying, and fixed patterns like 「ことができる」 and 「ことがある」, take only 「こと」.
With like, dislike, good at and bad at, 「の」 is the usual choice.