Keywords: why-not, let's
1. Used alone in response to someone else's suggestion to do something to mean that you agree and can see no reason why you should not do it. English = 'Why not?'
1. Used to introduce a suggestion to your partner to do something in a polite and friendly way. English = 'Why don't (you , we, etc.)..' 'Let's just do ....'
2. Used in 'self-talk' (signing to oneself or telling someone else what you thought) to express the sudden arrival of a desire to do something especially since you could think of no good reason why not to do it. English = "Why don't I....?" "Why shouldn't I...? "Let me just...."
1. Used to ask a question about the reason or cause of something not happening, especially when you expected someone to have done something. English = Why not? Why didn't....?