Breakdown of Donne-lui un frein, n'importe lequel.
un
a / an (masculine)
lui
him / to him / to her (indirect object)
donner
to give
le frein
the brake
Questions & Answers about Donne-lui un frein, n'importe lequel.
Does Donne-lui un frein mean "give him a break" (as in "cut him some slack")?
No, this is a trap caused by English homophones! A frein is a literal physical "brake" (like on a bicycle or a car). To say "give him a break" in the sense of being lenient, you'd use a phrase like Sois indulgent avec lui or the informal Lâche-lui la grappe. If you mean a break to rest, you'd use une pause.
Why do we use n'importe lequel here instead of n'importe quoi?
N'importe quoi means "anything" in a general, abstract sense. Here, you aren't saying "give him anything in the world," you are saying "give him any one (out of a specific category of items)." When "any" refers back to a specific noun—in this case, un frein—you must use n'importe followed by a form of lequel.
How do I know whether to use lequel, laquelle, or lesquels?
Why use lui instead of le, and why is there a hyphen?
The verb structure is donner quelque chose à quelqu'un (to give something to someone). Lui is the indirect object pronoun meaning "to him" or "to her". If you used le, it would mean "Give it". The hyphen is required because in affirmative commands (the imperative mood), object pronouns move to the right of the verb and are attached with a hyphen.