Use just for something that happened a moment ago. It goes after have / has, before the past participle.
just
I've just eaten.
She's just left.
We've just come home.
Try it: Man: Is there any coffee? Woman: Yes, I've ___ made some. It's still hot. Choose what the woman says. (yet / just / never) Answer: just — The coffee is still hot, so she has just made it.
already
Use already for something that happened sooner than you thought. It also goes after have / has.
She's already finished.
I've already paid.
They've already gone home.
Try it: Man: I finished the book you gave me. Woman: You've ___ read it? I gave it to you this morning! Choose what the woman says. (already / ever / yet) Answer: already — He read it sooner than she thought, so she says already.
yet: questions and negatives, at the end
Use yet at the end of a question or a negative sentence, for something you think will happen.
Have you eaten yet?
Has the bus come yet?
We haven't seen it yet.
Try it: Man: I made soup for you. Have you tried it ___? Woman: No. Give me a minute. Choose what the man says. (just / yet / before) Answer: yet — He asks about something he thinks she will do soon, so his question ends with yet.
American English also says I already ate
In American English, you can also use the past with these words: I already ate. Did you eat yet? Both ways are okay.
I just saw her.
Did you see it yet?
We already paid.
Try it: Man: The dentist closes at five. Did you call ___? Woman: Yes, this morning. I'm going on Friday. Choose what the man says. (ago / yet / never) Answer: yet — He asks if she called before now, so his question ends with yet.
It happened, and you can see it now
Use have / has + past participle when something happened and you can see the result now.
Oh no, I've broken my glasses.
Look, someone has opened the window.
Try it: Woman: Why are you so happy? Man: I've ___ my keys! They were in my coat. Choose what the man says. (lost / found / washed) Answer: found — He has his keys now, so he has found them.