Learn Swedish: Hon nickar inte med huvudet. - She does not nod her head.

In Swedish, actions involving moving a body part often use the preposition med (with). The phrase translates literally to nodding with the head. You will also see this in similar phrases like peka med fingret (point one's finger).

Why does the sentence use huvudet (the head) instead of hennes huvud (her head)?

When it is obvious whose body part is being used, Swedish prefers the definite form of the noun rather than a possessive pronoun. Since she is the one nodding, Swedish assumes it is her own head.

Do I always have to add med huvudet when someone nods?

No, you can simply say Hon nickar inte. Just like in English, the verb nickar already implies using the head. Adding med huvudet is completely optional but sounds very natural.

Could I put inte before the verb, to match the English does not nod?

No, in a standard main clause, Swedish strictly follows the V2 rule where the conjugated verb comes second. Hon is the first part of the sentence, so the verb nickar must come next, pushing inte to the third position.