Just is used with a verb to indicate the immediate past.
- They have just gone. (i.e. They went a very short time ago.)
Just can also emphasise the idea of at the present or close to the present.
- ‘Where is my tea?’ ‘I am just going to make it.’
- I was just about to tell you.
Note that just now can mean either at this moment or a few moments ago, depending on the tense.
- I am busy just now. (at this moment)
- Tom was here just now. (a few minutes ago)
Just can mean only.
- Complete set of garden tools for just $19.99!
- He is just an ordinary man.
In some contexts the meaning is more like scarcely.
- We just caught the train.
- I arrived just in time.
This meaning can be emphasised by only.
- I have got only just enough money for a cup of coffee.
Just often means exactly.
- It is just two o’clock.
- This is just as good as that.
- Just then, we heard a strange noise.
When just means a moment ago, a present perfect tense is most common in British English.
- ‘Where is John?’ ‘He has just gone out’.
- I have just had a call from Alice.
In American English, a past tense is common in this case.
- ‘Where is John?’ He just went out.
- I just had a call from Alice.
When just now means a moment ago, it is used with a past tense in both British and American English.
- I heard a strange noise just now.