Learn Swedish: Lars klipper sin häck.

Sin is a reflexive possessive pronoun. Because Lars is the subject of the sentence, and he is trimming his own hedge, we must use sin. If we used hans (his), it would mean Lars is trimming some other man's hedge.

Why do we use sin specifically, rather than sitt or sina?

The choice between sin, sitt, and sina depends on the noun being owned. Häck (hedge) is an en-word and it is singular here (en häck), so we use the en-word form: sin.

Why is it sin häck and not sin häcken?

In Swedish, whenever you use a possessive pronoun (like min, din, hans, or sin), the noun that follows it must always be in its indefinite form. So it is sin häck, even though in English we are talking about a specific hedge.

Does the verb klipper only apply to gardening?