fermer la fenêtre

You need the verb fermer. It is a regular -er verb. This makes it very easy to conjugate. For a quick command, say Ferme la fenêtre !. If you are asking politely, use a question. Try Est-ce que tu peux fermer la fenêtre ?. Don't forget the article la. Window is a feminine noun in French.

Use it when the winter wind bites. Use it when the city noise is too much. It is perfect for a stuffy office. You can use it in a car too. If you are at a friend's house, ask first. It is a polite way to control your environment. Use it when you leave your house for safety.

Avoid using it for "closing" a business deal. That would be conclure une affaire. Don't use it for "closing" a book. Use fermer le livre for that. Never use it for "closing" your mouth rudely. That would be Tais-toi. It is strictly for physical or digital windows. Do not use it for closing your eyes either.

In France, the courant d'air is a legendary enemy. Many French people fear a draft will cause a cold. You will see people closing windows even in summer. Shutters, or les volets, are also a big deal. You close the window, then the shutters for total privacy. It is a ritual of the French evening. It signals that the day is officially over.

You might hear fermer la fenêtre à clé. This means locking it with a key. Another one is fermer la fenêtre en grand. This is actually a mistake; you ouvrir en grand. For closing, you might say fermer bien la fenêtre. This means closing it tightly. You can also say fermer l'onglet for a browser tab.