Do I understand right？
I saw a sentence on the book said" i will send you the letter in 3 days", it means:the letter will be sent at anytime during the following 3 days?

Question

Do I understand right？
I  saw a sentence on the book said" i will send you the letter in 3 days", it means:the letter will be sent at anytime during the following 3 days?

Answer

No... "in three days' means "after three days."  If today is Monday and I say that I will send you the letter in three days, it means I will send the letter on Thursday.

If I say "I will send the letter 'within three days,' I mean that I will send it at any time during the following three days.