Чем отличается just от only?

Well, most grammar books are slow to register informal language, and the use of
'just' as a synonym for 'only' is not so recent by now, but is still a bit
informal.

Anyway, 'just' is used as a synonym for some, but not all, uses of 'only',
e.g., you would be saying different things in:
1. Jane was the only daughter of King Henry.
2. Jane was the just daughter of King Henry (as opposed to the unjust
and corrupt Princess Ann).

In general, 'just' is synonymous with 'only' as an adverb, not as an adjective.

In some collocations, 'just' replaces 'only', rather than being /just
/synonymous. Compare:
3. Just a minute.
4. Only a minute.
5. ..rather than being just synonymous.
6. ..rather than being only synonymous.

In 5 'just' is in fact closer in meaning to 'merely' than to 'only'. There are
also other cases in which, in the process of grammaticalization, there has
developed some differentiation in meaning between the two words, e.g.:
7. It just occurred to me...
8. It only occurred to me...
9. It only now occurred to me...

Compare also "...not just..." with "Not only...", etc. etc. etc.

Where the two can be used interchangeably, the difference is not one of
grammar, but possibly one of register. 'Just' is usually used in an informal
and colloquial register, while 'only' can more freely be used in more formal
settings. This difference has by now become much less marked than it used to be
though, if anything is left of it at all. Compare:
10. I would only like to add that...
11. I would just like to add that...

Last, but not least, 'just' is /only/just/ one syllable long, while 'only' is
two syllables long. This is a difference any poet or lyrics writer should take
into account.

Only от слова one, то есть, это родное слово в этом германском языке, а just заимствовано из латыни.
Посему у них разница в употреблении.

Just - больше используется как междометие в смысле "только, просто"

Ну, я только хотел спросить - Well, I just wanted to ask...

Only - в значении "только", полностью исключающем другие варианты. "Только я (один я) могу это сделать" - Only I can do this.

Дайте полностью предложение, в котором у Вас используется just is

just IS = просто есть.
just (adverb) в большинстве контекстов можно без потери смысла заменить на simply, а only на exclusively.
just = только (просто)
only = только (исключительно)

Вот смотри: Конструкция "Просто сделай это" - это just. А конструкция "Сделай только это" - это only

Just только 5 минут, промежуток времени обычно, слово замена - просто. give me just 5 minutes - просто дай мне 5 минут. Дай мне только минут.
А Only это об уникальности. Only you - только тебе, тебя. О чем то исключительном
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