How do I know whether to use 'depuis' or 'pendant'?

The words 'depuis' and 'pendant' are both used to express the duration of an event, meaning that it is easy for English speakers to mix them up and use them incorrectly. However, there is an important distinction between them which will make it easier to determine which one to use. 'Depuis' means "since" or "for", and is always used with a verb in the present tense. 'Depuis' is used to talk about an action in the past that is still continuing today. For example, if you wanted to ask someone how long they had been studying French for, you would ask "depuis quand étudiez-vous le français"? Depuis is used here as the person is still studying French, and so the event of studying French has not ended. See also that the present tense of étudier has been used in the vous form in order to agree with the use of depuis. On the other hand, 'pendant' is used to describe the duration of an event with a clear end date, that has no link to the present day. Therefore, if you wanted to ask someone how long they had studied French for, you would ask "pendant combien de temps avez-vous étudié le français?". Pendant is used here because the act of studying French has finished, and therefore is a completed action that is no longer occurring. You should also be aware that pendant can be used to mean "during" when it is followed by a noun. For example, if you wanted to say "I saw lots of tourists during my stay" you would say "J'ai vu beaucoup de touristes pendant mon séjour". To summarize, 'depuis' is used to refer to an ongoing event, and is used with the present tense, and 'pendant' is either used to refer to refer to a finished event, or is used to describe something that happened during an event.