Just and only can mean the same thing. But sometimes just and only cannot be used in the same way. For example, just can also mean "correct", but that is when it is an adjective. Just can also mean recently You can say, for example: "I only need to go to school once a week." or "I only need to go to school once a week". You can say "I just finished school" (I recently finished school), but "I only finished school" means in fact that you did nothing but finish school. You can say "The Judge's ruling was just" (here, just is an adjective which means something similar to correct). You cannot say "The Judge's ruling was only", because only is an adverb. A good test to see if you can say both or just one is: 1. replace just or only with recently. If recently works, you cannot say only. 2. replace just or only with simply, if simply works, you can say both. 3. replace just with an adjective (for example: good), if it works, you cannot say only. In conclusion, just has many meanings but "only" has one.

Most of the time these are interchangeable. There are some times when convention has a speaker use one and not the other: I just can't believe this! this is the only time I will say this. Only time will tell.