Just and Only also have the same meaning of “no more than ~”
I just had coffee for breakfast, so I’m really hungry.
I only had coffee for breakfast, so I’m really hungry.
I just need 20 minutes more to finish this report.
I only need 20 minutes more to finish this report.
That pen costs just $1? I’ll take two please!
That pen costs only $1? I’ll take two please!
Only is also used as an adjective meaning “the single one of its kind:”
This is the only computer I own.
He took my only pen, now I have to buy one.
Jack is an only child.
Just is also used to mean a little while ago or in the immediate past (recently):
I just woke up, so I’m still sleepy.
I just heard about your accident. Are you ok?
I’m not hungry because I just ate lunch.
That couple just got married and are going on their honeymoon.
(http://blog.myhappyenglish.com/2013/11/15/free-confusing-english-vocabulary-lesson-just-vs-only/ )
Just is also used to mean "barely, hardly, or almost not":
I just passed the exam.
I barely passed the exam.
I just had coffee for breakfast, so I’m really hungry.
I only had coffee for breakfast, so I’m really hungry.
I just need 20 minutes more to finish this report.
I only need 20 minutes more to finish this report.
That pen costs just $1? I’ll take two please!
That pen costs only $1? I’ll take two please!
Only is also used as an adjective meaning “the single one of its kind:”
This is the only computer I own.
He took my only pen, now I have to buy one.
Jack is an only child.
Just is also used to mean a little while ago or in the immediate past (recently):
I just woke up, so I’m still sleepy.
I just heard about your accident. Are you ok?
I’m not hungry because I just ate lunch.
That couple just got married and are going on their honeymoon.
(http://blog.myhappyenglish.com/2013/11/15/free-confusing-english-vocabulary-lesson-just-vs-only/ )
Just is also used to mean "barely, hardly, or almost not":
I just passed the exam.
I barely passed the exam.