both 'just' and 'only' have several meanings or uses 1. just = exactly or precisely; no more than; merely; quite; really. it is just the one I want... he is just fast enough to qualify... she is just using a translator 2. only (adjective) = sole: he is the only child 3. only (adverb) = merely solely, no longer ago than, no better than: I can only guess... it was only yesterday... it is only average cooking.... 4. only (conjunction) = however, except that, if not: I could give many examples, only it takes too much time

Check this out: http://www.espressoenglish.net/difference-between-just-and-only/

Just The word “just” has several possible definitions: 1) Recently Be careful – I just washed the floor, and it’s still wet. (= I washed the floor a few minutes ago) He just finished a big project. (= he finished the project very recently) 2) Only I have just one brother. (= I have only one brother) I thought you were hungry, but you ate just half of your sandwich. (= you ate only half of your sandwich, and no more) Only The word “only” can be replaced with “just” in most situations: Only two students came to class on the day before Christmas. = Just two students came to class on the day before Christmas. My kids only use the internet for schoolwork, not for playing games. = My kids just use the internet for schoolwork, not for playing games. In the expression “If only…” you can use “just” if you change the structure a little bit: If only I had studied harder. I would’ve passed the test. = If I had just studied harder, I would’ve passed the test. “Only” and “just” are interchangeable with definition 2 of “just,” but not with definition 1. Definition 2 – Same meaning We have just one daughter. = We have only one daughter. Definition 1 – Different meanings I just washed the floor (a few minutes ago) I only washed the floor (and I didn’t wash the table) However, it also depends on the context: “Did you clean the whole house?” “No, I just washed the floor” (= I only washed the floor). “Why is the floor wet?” “Because I just washed it” (= I recently washed it. In this case, you can’t use “only”) Word order When you use “just,” the word order matters: I just ate two pieces of pizza. ( = I recently ate two pieces of pizza) I ate just two pieces of pizza ( = I ate only two pieces, not 3 or 4 or 5)

Vou tentar ajudar em alguns exemplos: Just, é usado numa forma mais simples para algo que se queira naquele momento Ex: I'm just want to say, hello again. (Eu só queria dizer, olá novamente) E only, você pode usar para representar uma única coisa Ex. He has only one son. (Ele só tem um filho/Ele tem um único filho) Estou aprendendo ainda, espero ter ajudado.