(Note: I'm an English teacher from the USA - what I'm going to say may not be the same in England, Australia, etc.!)
Originally Posted by Petra J.
As a previous poster noted, couldn't is most often used as the past of can't.
It is difficult to explain what's really going on here without more context from you. Modals in English are often used to express different attitudes, in addition to probability or possibility - which is sometimes really difficult for non-native speakers to get. Note these two examples, which both discuss present tense situations:
She can't be in France! I need her help with my presentation today! (expresses disappointment or surprise that she is in France)
She couldn't be in France - she doesn't have a visa to enter the country. (expresses improbability or impossibility that she is in France)