How do you say can't in French?
On this page, we look at how to say "can't" in French. We will assume that you are familiar with the section on how to say "can" in French. If you are not, then it is recommended that you look through that section first before continuing with this one.
The word "can't" (and indeed the formal variant cannot) is basically the negative of "can". In French, there's no special word as such: we need to combine the appropriate word for can with the general formula for making a negative sentence in French.
You'll recall from the section on how to say "can" in French that the French for "can" is usually an appropriate form of the verb pouvoir. And if "can" has the meaning of "have the know-how to...", then the usual translation is savoir instead of pouvoir.) So, for example, in order to say I can't come tonight, the steps to forming this sentence are to find the French for I can come tonight, and then make this sentence negative.
Recall that the French for I can-- i.e. the je form of pouvoir-- is je peux. So I can come tonight would be je peux venir ce soir. So far so good.
Then, we need to make this sentence negative. From the section on forming negative sentences in French, you'll recall that the usual way is to put ne ... pas around the verb. Specifically, we need to put it around the main verb-- in other words, not the infinitive that comes after the form of pouvoir, but the form of pouvoir itself. This means that in our example, it goes around peux and we end up with the sentence: Je ne peux pas venir ce soir for "I can't come tonight".
Turning to another example: suppose we want to say "She can't swim". Here, there are two choices of verb. If we mean "can't" in the sense of "doesn't have permission", then French speakers would usually use pouvoir as before. But in the usual sense of "doesn't know how to", then savoir would be the usual verb.
As before, the trick is to construct the sentence She can swim and then negate it. Recalling that "She can" in French (in the sense of "She knows how to...") is Elle sait, the elle form of savoir, then this gives us the sentence Elle sait nager as our starting point. Then, we negate this, to give us Elle ne sait pas nager as the French for She can't swim.
Omission of ne when saying "can't"
As with negative sentences in general, in relaxed, everyday spoken French, it is common to omit the word ne. So "She can't swim" in informal spoken French would in reality often be Elle sait pas nager. However, note that an informal form such as this only sounds natural if spoken with some fluency. So to begin with, it is probably advisable to stick to inserting the ne even if it some cases it would be slightly formal or emphatic when used by a native speaker. (Put another way: if you insert the ne, nobody will accuse you of "getting it wrong".)