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Turns a verb into a noun

Turns a verb into something a sentence can treat as a noun: learning, finding, settling, going together. It is the piece inside several extremely common frames — "before doing", "because", "hard to do", "only if" — and those frames require this form specifically. Where the alternative 는 것 packages up a whole concrete situation, this points at the activity itself in the abstract. In speech it often contracts with a following topic marker into a single syllable.