Can you please tell me if all the sentences are grammatically correct?

1) Yesterday something really funny and unusual happened to me at the swimming pool
2) I was swimming when I saw a guy that was climbing the diving board's stairs (?). It was very convincing and he jumped the last two steps.
3) He prepared his dive and started to run but he fell over the board.
He fell into the water. When I saw him out of the pool, his back was totally red.
4) I know that he got hurt but that was very funny.

Thanks

#1 is fine.

#2 - I was swimming when I saw a guy climbing the ladder to the diving board. It was very convincing and he jumped the last two steps. Notice corrections in the first sentence. Please explain the second sentence; I don't understand it.

#3 - He prepared for his dive and started to run, but he fell off the board and into the water. When I saw him get out of the pool, his back was totally red. Notice all the corrections.

#4 - I know that he got hurt, but that was very funny. Notice the correction.

How do you write a proper stanzas?

and how many lines are in a stanza?

Yes, all the sentences you provided are grammatically correct. However, there are a few suggestions for improving the clarity and flow of the sentences:

1) Yesterday, something really funny and unusual happened to me at the swimming pool.

2) I was swimming when I saw a guy climbing the stairs of the diving board. He climbed very convincingly and jumped the last two steps.

3) He prepared for his dive and started to run, but he fell off the board and into the water. When I saw him come out of the pool, his back was totally red.

4) I know that he got hurt, but it was still very funny.

These suggestions help to make the sentences more concise and better organized for readability.