just triggers the use of perfective aspect (have + -en) when it marks a recently accomplished prior action.
- I had just finished eating when the doorbell rang.
- I've just left work. I should be back home in an hour.
In both the UK & the US the past perfect is used with just to mark a recently accomplished prior past action.
In American English the simple past is frequently used instead of the present perfect.
- just now = precisely now.
- just then = à ce moment précis
just can qualify a preposition in many of the same ways "right" can:
- The boss always naps just after lunch.
- He usually makes weak coffee, but this cup is just right.
- She smelled the fire just in time to still be able to put it out.
- They live just over the hill there.
"just" is often used when making a suggestion, when criticizing, when explaining one's intentions/reasoning...
- I'm just wondering if we should even continue producing these books, we keep losing money on them.
- just saying (je dis ça, je dis rien)
- I just wanted to say thank you!
- I just thought it wasn't a big deal. I didn't realize it was so important.