express need or obligation

the essential semi-auxiliary verb devoir, which means "must", "have to", or "need to" when placed before another verb.

• Je dois manger plus de légumes. — I must eat more vegetables.
• Ils doivent acheter plus de livres. — They need to buy more books.

Remember that the impersonal expression il faut + infinitive can also express a need or obligation.

• Il faut manger. — It is necessary to eat.
• Il nous faut payer des taxes. — We have to pay taxes.

You can also use the impersonal construction from the last unit, il est + adj + de.

• Il est nécessaire de reprendre le travail. — It is necessary to return to work.
• Il est nécessaire de faire attention. — It is necessary to pay attention.

Another way to express obligation is avoir à, though this is rarely used by French speakers because it tends to create vowel conflicts.

• Je sais ce que j'ai à faire. — I know what I have to do.
• J'ai quelque chose à vous dire urgemment. — I have something to tell you urgently.

What about when you want to say that you need something (instead of having to do something)? One way you learned previously is to use il faut with a noun instead of a verb.

• Il faut du lait. — Milk is needed.
• Il faut un début à tout. — A beginning is needed for everything.

A more common expression for need is avoir besoin de quelque chose. While this literally translates as "to have need of something", a better translation is "to need something".

• J'ai besoin d'un stylo. — I need a pen.
• Il a besoin d'eux. — He needs them.
• Elles ont besoin de magazines en anglais. — They need magazines in English.
• Vous avez besoin de ce produit. — You need this product.

You can also use this expression with verbs.

• Vous avez besoin de gagner plus d'argent. — You need to earn more money.
• Mes amis ont besoin de manger. — My friends need to eat.

Notice that besoin is invariable in this expression, but the noun besoin ("need") is just a standard masculine noun that does have a plural form.

• Il a des besoins importants. — He has important needs.
• Il y a un besoin urgent d'agir. — There is a pressing need to act.

Consider the difference between "I don't have to" and "I must not". The former expresses a lack of obligation, while the latter expresses an obligation to avoid an action. In French, to express a lack of obligation, use a negation with avoir besoin de or avoir à.

• Je n'ai pas besoin d'un stylo. — I don't need a pen.
• Nous n'avons pas besoin de votre permission ! — We don't need your permission!
• Elle n'a pas à parler. — She doesn't have to speak.
• On n'a pas à manger maintenant. — We don't have to eat right now.

To express "must not" in French, use a negation with devoir or il faut.

• Elle ne doit pas manger de poisson. — She must not eat fish.
• Nous ne devons pas nous mentir. — We must not lie to each other.
• Il ne faut pas réfrigérer les tomates. — One must not refrigerate tomatoes.
• Il ne faut jamais oublier les leçons de l'histoire. — We must never forget the lessons of history.

https://www.duolingo.com/skill/fr/Abstract-Objects