The word 'just', when used as an adverb, has three meanings: 1. Very recently 'I've just heard the news'. 2. Exactly 'That's just what you need'. 3. Only 'He just speaks English.' When 'just' is used in the third meaning, there is no difference between 'just' and 'only'. You can replace 'just' with 'only' in sentence #3 : 'He only speaks English', but you can't use 'only' in either of the other two sentences.