A few days ago I heard an English-speaking politician say in French:

On ne peut pas prendre le gouvernement sérieusement dans ce dossier.
We can not take the government seriously in this matter.

There is a common and understandable mistake here. In fact, this is a particularly subtle example of what are called false friends or faux amis. These are words that look alike in both languages but have different meaning or uses.

In this particular case, the proper wording should have been:

On ne peut pas prendre le gouvernement au sérieux dans ce dossier.

Why “au sérieux” and not “sérieusement”?

This is tricky.  “Sérieusement” means “seriously” or “in a serious manner” and is very often used with two verbs,”parler” (talk) and “”penser” (think),  as in:

Il faut qu’on se parle sérieusement. (We have to talk seriously.)
Pensez-y sérieusement avant de prendre une decision. (Think about it seriously before making a decision.)

So far, these uses resemble the English “seriously.”  But “to take seriously” in the sense of “to consider of a serious nature” is best translated by “prendre au sérieux.” Here are some examples:

C’est une menace qu’il faut prendre au séreux. (This is a threat to be taken seriously.)
Il ne faut pas prendre les choses trop au sérieux. (Don’t take things too seriously.)
Le réchauffement climatique est à prendre au sérieux. (Global warming is to be taken seriously.)

So, anytime you want to say “take seriously,” use “prendre au sérieux” and not “prendre sérieusement” unless there is some subtle nuance.

More false friends: “à la légère” and “légèrement” are not the same

The opposite of “prendre au sérieux” is “prendre à la légère” (take lightly or slightly) and not “prendre légèrement.”  Here are some examples.

C’est une menace qu’il ne faut pas prendre à la légère. (This is not a threat to be taken lightly.)
Il ne faut pas prendre les choses à la légère. (Don’t take things too lightly.)
Cette équipe n’est pas à prendre à la légère. (This team is not to be taken lightly.)

This is not to say that “prendre légèrement” does not exist. Just think twice before using it. In fact, it is actually used quite a bit when giving driving instructions to a destination, but this has nothing to do with the common usage here.