Learn Dutch: Zij grijpt hem bij de pols. - She grabs him by the wrist.

This sentence uses a structure called the possessive shift. When a physical action happens to someone's body, Dutch prefers to make the person the object of the action (hem) and use a definite article (de) for the body part. It emphasizes that he is the one being grabbed, and the wrist is just the location.

Could I just say Zij grijpt zijn pols?

It is grammatically correct, but it sounds less natural for a physical interaction. Saying zij grijpt zijn pols treats the wrist as an independent object, much like grabbing a phone or a bag. Using grijpt hem bij de pols makes it clear that the action is directed at the person himself.

Why use the preposition bij here instead of op or aan?

The preposition bij is used to indicate the specific point of contact where you seize or hold someone, very similar to 'by' in English. You typically use op for a surface impact, like a pat on the shoulder (op de schouder), and aan for pulling or leading, like holding someone by the hand (aan de hand).

In this sentence structure, is hem the direct object, or is de pols the object?