How do you say they are serious in french?

There are a few ways to say "they are serious" in French, depending on the context and nuance you want to convey:

Formal:

* Ils sont sérieux. (This is the most direct and neutral translation.)

* Ils prennent les choses au sérieux. (This emphasizes their seriousness about something specific.)

Informal:

* Ils sont sérieux, hein? (This adds a bit of surprise or incredulity.)

* Ils font pas de blague. (This means "they're not joking" and implies they are serious.)

* Ils ne rigolent pas. (This means "they don't laugh" and conveys a sense of seriousness or solemnity.)

Other options:

* Ils sont très sérieux. (They are very serious.)

* Ils sont à prendre au sérieux. (They are to be taken seriously.)

Choose the phrase that best fits your situation and the level of formality you want to achieve.