Le corps, en détail

You met the rule already: French puts the definite article on body parts where English uses a possessive. Now that you have most of a body, here is where it gets its full workout.

The daily routine

Almost everything you do to yourself takes a reflexive verb plus a definite article:

• Je me lave les mains. I wash my hands.
• Je me brosse les dents. I brush my teeth.
• Il se coupe les cheveux. He cuts his hair.

The me and se say whose. The article stays definite. Je lave mes mains is not so much wrong as audibly foreign, and it is the single most reliable giveaway in a learner's spoken French.

Injuries work the same way

Il s'est cassé la jambe. He broke his leg. Note that it is his own leg, said with a reflexive and an article, and that the past participle does not agree here even though it looks as though it should. La jambe is the direct object and it follows the verb, so cassé stays as it is.

If you break somebody else's leg, the reflexive goes and a pronoun arrives: il lui a cassé la jambe.

Two words for hair

Cheveu is a single hair on your head, and you almost always want the plural: il a les cheveux noirs. Poil is body hair, and an animal's coat. Confusing them is a memorable mistake rather than a serious one.

Saying where it hurts

Avoir mal à plus the article, contracted where needed: au dos, aux dents, à la gorge. Souffrir is to suffer and is stronger, used for conditions rather than a headache. Blesser is to wound or injure, and se blesser is to hurt yourself.

Faire mal

Ça fait mal is that hurts, said of a thing doing the hurting. J'ai mal is I am in pain, said of the person having it. Both are common, and they are not interchangeable: je fais mal means you are the one causing pain to somebody else.