Need help on "can/peux" usage in French
I got confused when to use "peux". There are some question that starts with "can" in English but in French version it doesn't include "peux". How do I know when to use peux or not? For example: Can you turn off the light? - Tu peux éteindre la lumière? - Tu éteins la lumière? Is there a rule on "peux" usage or both sentence can be used always?
Hmm, tricky. Pouvoir tends to make the sentence more formal in my opinion, much like 'would you be able to...' in English (-> Pouriez-vous... [super formal, don't use that])
Where the confusion arises is in that sometimes the future is expressed with the present in French. "Demain, je vais au marché" -> "I go to the store tomorrow" but is taken to mean "I will go tomorrow". We do the same in English: I'm going to the store (tomorrow), need anything? In French, the true future would be "je vais aller/j'irai demain"
This means that the future tense doesn't explicitly have to be stated in questions that would otherwise have the future tense in English. "Hey, when you go, will you turn off the light?" Uses future tense and I would wager is less formal than "can/could you turn off the light" which uses pouvoir.
Therefore, questions that we would use future tense for can be accomplished with the present without pouvoir "Tu éteins la lumière?" Would mean, "are you going to turn the light off?" because of the conflation of present and future. But not, could you do it for me?
tl;dr. Pouvoir is more formal. like "could you do something for me"
Using present tense is like a simple future tense question in English. "You gonna turn off the light"?
I'd say the opposite, peux-tu is too formal for regular speech. Even with a waiter just use est-ce que if you need to (with vous naturally), but with friends, just use the rising inflection. Good video on it here by a native french speaker. https://www.youtube.com/watch?v=ohTNjMUDdwU see 4:10 & 5:00