Can you please check these sentences for me? Thank you very much.

1) Storm wants to borrow Rob's bicycle pump because she wants to train for the triathlon.
2) In the beginning she didn't want to race (for the triathlon) but then she changed her mind. She was slow on her bike but she's been working on her cycling recently and she is much faster now.
3) Johnny really thinks she has a good chance to win the race.
Rob is going to train on the route of the race but he doesn't really need to check a map. Rob doesn't give Johnny his pump.
4)Rob's group is in the computer room though it should be Storm's computer session.
5) Rob swapped the session without asking Storm for permission because he wants to train for the triathlon this afternoon.
6) Storm gets all worked up because she doesn't want to put her session off until the afternoon.
7) Rob asks her to do beach activites instead but she refuses.
8) Rob tells her that he wrote his name on the noticeboard and warns her to check always the noticeboard before a session.

Commas are needed in 1 and 2 and 3 and 7. You should be able to find those places by now.

http://grammar.ccc.commnet.edu/grammar/commas.htm
See especially #s 2 and 3.

Everything else is fine.

Sure, I'd be happy to check these sentences for you. Here are the corrected sentences:

1) Storm wants to borrow Rob's bicycle pump because she wants to train for the triathlon.

2) Initially, she didn't want to participate in the race for the triathlon, but then she changed her mind. She used to be slow on her bike, but she has been working on her cycling recently and is much faster now.

3) Johnny truly believes that she has a good chance to win the race. However, Rob is going to train on the route of the race and doesn't really need to check a map. Rob doesn't lend his pump to Johnny.

4) Although it should be Storm's computer session, Rob's group is in the computer room.

5) Rob swapped the session without asking Storm for permission because he wants to train for the triathlon this afternoon.

6) Storm becomes agitated because she doesn't want to postpone her session until the afternoon.

7) Rob suggests doing beach activities instead, but she refuses.

8) Rob informs her that he wrote his name on the noticeboard and advises her to always check the noticeboard before a session.

Please note that these edits and corrections may vary depending on the context or intended meaning. Additionally, it's always a good idea to double-check grammar and clarity by proofreading your sentences before finalizing them.