Use am, is, are and the -ing form to talk about a future plan you have already arranged, with a future time word like tomorrow, tonight, next week.
A plan: am/is/are + -ing
I'm flying to London next Tuesday.
They're having a meeting at 3 PM.
My parents are visiting us this weekend.
Not "I fly to London next Tuesday." — say "I'm flying to London next Tuesday.". For a plan you have arranged, use am/is/are and -ing: I'm flying.
Try it: He ___ his new job next Monday. (is starting / starts / start) Answer: is starting — A new job next Monday is an arranged plan, so use is and the -ing form: He is starting. "Starts" is for habits or timetables, and "start" is missing the is.
A plan you are not doing
For a plan you are not doing, put not after am, is, are: 'm not, isn't, aren't and the -ing form.
We're not working on Friday afternoon.
She isn't coming to the party tonight.
I'm not meeting them tomorrow.
Not "We don't working on Friday." — say "We're not working on Friday.". The not goes after are: we aren't working, never we don't working.
Try it: Sorry, we ___ on Friday afternoon, the office is closed. (aren't working / don't working / not working) Answer: aren't working — The not goes after are: we aren't working. "Don't working" uses the wrong helper, and "not working" is missing the are.
Asking about a plan
To ask about a plan, put am, is, are before the person. Add a question word like what or where at the start to ask for details.
Are you leaving tomorrow?
Is Mei coming to the party tonight?
What time are you leaving?
Where are we meeting them?
Not "You are leaving tomorrow?" — say "Are you leaving tomorrow?". To ask, put are before you: Are you leaving?
Try it: ___ Mei coming to the party tonight? (Is / Does / Will) Answer: Is — For an arranged plan, ask with is and the -ing form: Is Mei coming? "Does Mei come" asks about a habit, and "Will Mei come" is a different way to talk about the future, not this arranged-plan form.
Try it: Amina ___ Lars tomorrow, but she ___ him on Friday. (is meeting … isn't seeing / meets … doesn't see / is meeting … doesn't see) Answer: is meeting … isn't seeing — Both are arranged plans, so both use is and the -ing form: she is meeting Lars, and she isn't seeing him on Friday. The present simple (meets, sees) is for habits, not for one arranged plan.