What is the difference in meaning between remember to post a letter and posting letter?

"Posting a letter" refers to the actual physical act of dropping a letter into a mailbox or sending it through the postal system. In other words, it's the action of mailing the letter.

In contrast, "remember to post a letter" is a reminder or a future intent to perform the act of posting a letter. It's a reminder to oneself to take action and send a letter in the future.

So, the main difference between the two lies in the timing of the action. 'Posting a letter' refers to the action happening now or in the past, while 'remember to post a letter' is a reminder to do it at a later time.