be doing / be going to do

What are you doing tomorrow?

We use am/is/are + -ing (present continuous) for something happening now:

‘Where are Sue and Amanda?’ ‘They‘re playing tennis in the park.’
Please be quiet. I‘m working.

We also use am/is/are + -ing for the future (tomorrow/ next week etc.):

Andrew is playing tennis tomorrow.
I‘m not working next week.

I am doing something tomorrow = I have arranged to do it, I have a plan to do it:

Sophie is going to the dentist on Friday. (= she has an appointment with the dentist)

We‘re having a party next weekend.
Are you meeting your friends tonight?

What are you doing tomorrow evening?

I‘m not going out tonight. I‘m staying at home.

Be careful! Do not use the present simple (I stay / do you go etc.) to say what somebody has arranged to do:

I‘m staying at home this evening. (not I stay)
Are you going out tonight? (not Do you go)
Lisa isn’t coming to the party next week. (not Lisa doesn’t come)

But we use the present simple for timetables, programmes, trains, buses etc. :

The train arrives at 7.30.
What time does the film finish?

Compare:

present continuous (usually for people)

I’m going to a concert tomorrow.
What time are you leaving?

present simple (for timetables, programmes etc.)

The concert starts at 7.30.
What time does your train leave?

I’m going to …

I am going to do something

= I have decided to do it, my intention is to do it:

 

I‘m going to buy soine books tomorrow.
Sarah is going to sell her car.
I’m no t going to have breakfast this morning. I’m not hungry.
What are you going to wear to the wedding next week?
‘Your hands are dirty.’ ‘Yes, I know. I‘m going to wash them.’
Are you going to invite Martin to your party?

Something is going to happen

= we can see now that it is sure to happen:

Look at the sky! It‘s going to rain.  (black clouds now → rain)

Oh dear! It’s 9 o’clock and I’m not ready. I‘m going to be late.
(9 o’clock now and not ready → late)