I forgot to include these last sentences. Could you check if the sentences are OK? Thank you.

1)Where has Peter gone? There he is.
I've never flown in an plane before.
2)If don't run, you'll arrive late.
She'll buy herself a mobile phone, as soon as she has enough money.
3)She won't leave until she has finished her homework.
4) Jane is not here. She has just left for London. Have you been to the new supermarket yet?
5) I think I'll stay at home but I might also go out. Where do you think you'll go?

1) a plane OR an airplane

Sa

1) Where has Peter gone? There he is.

This sentence is grammatically correct. It is a question about the whereabouts of Peter, followed by spotting him.

I've never flown in an airplane before.
This sentence is also grammatically correct. It is a statement about not having any previous experience of flying in a plane.

2) If you don't run, you'll arrive late.
This sentence is grammatically correct. It is a conditional statement suggesting that running is necessary to avoid being late.

She'll buy herself a mobile phone as soon as she has enough money.
This sentence is also grammatically correct. It states a future action of buying a mobile phone, conditional upon having enough money.

3) She won't leave until she has finished her homework.
This sentence is grammatically correct. It expresses a condition for not leaving, which is the completion of homework.

4) Jane is not here. She has just left for London. Have you been to the new supermarket yet?
This sentence is grammatically correct. It consists of three separate statements. The first states that Jane is not present, the second indicates her recent departure for London, and the third is a question regarding someone's visit to a new supermarket.

5) I think I'll stay at home, but I might also go out. Where do you think you'll go?
This sentence is grammatically correct. It expresses two possibilities for the speaker's plans, followed by a question about the listener's intentions.