We use 'only' when we want to exclude other things or possibilities. There was only one piece left. He was an only child 'Just' can be used in most of the situations above, but it is also used for other meanings, meanings for which 'only' would not be used, or would have to be used within a phrase. He just missed the mailbox - He only missed the mailbox by a little. It is just 12 centimeters = It is exactly 12 centimeters. He just got here = He got here only a moment ago.

JUST has a few meanings... 1. JUST = "near now" My mom just called. I arrived at the airport just in time. 2. JUST = "similiar meaning to only" I only want to fall in love! I just want to fall in love! I have only one sister. I have just one sister.