Thank you very much for explaining to me the best choice.
I still have a few sentences I'm doubtful of. I included 7 doubts using question words. Thank you
1) Sometimes my parents aren't at home for (?) their work and so I have to stay at home alone with my brother.
2) I felt sorry for my mum because she has a broken leg (better: she broke her leg, she had a broken leg).
3) I finished primary school with an average mark of 9 out of (?) 10.
4) I was proud of myself on 24th July when I passed my driving test.
5) The teacher reprimanded me (= wrote a reprimand in the class register??)
6) It was the last sad moment in my life when my girlfriend broke up with me (?). While I was climbing a step I fell on the floor (?)(I fell down on the floor)
1) Sometimes my parents are at work, not at home, so I have to stay at home alone with my brother.
??
2) I felt sorry for my mum because she has a broken leg (better: she broke her leg, she had a broken leg).
I'd say "... because she broke her leg."
3) I finished primary school with an average mark of 9 out of 10.
OK
4) I was proud of myself on 24th July when I passed my driving test.
OK
5) The teacher reprimanded me (= wrote a reprimand in the class register??)
OK, either way you want to write it is fine.
6) It was the last sad moment in my life when my girlfriend broke up with me. While I was climbing a step, I fell down.
If you fall down, "on the floor" is usually assumed -- or "on the field" if you're outdoors, etc.
I don't understand "last sad moment" -- do you mean this? ~~> The last time I felt sad was when my girlfriend broke up with me.
1) Sometimes my parents aren't at home for work reasons and so I have to stay at home alone with my brother.
2) I felt sorry for my mum because she broke her leg.
3) I finished primary school with an average mark of 9 out of 10.
4) I was proud of myself on 24th July when I passed my driving test.
5) The teacher reprimanded me (most likely means a verbal reprimand or scolding).
6) It was the last sad moment in my life when my girlfriend broke up with me. While I was climbing a step, I fell on the floor (fell down on the floor).
1) Sometimes my parents aren't at home for their work and so I have to stay at home alone with my brother.
To figure out the correct word to use in this sentence, you will need to determine the purpose or reason why your parents aren't at home. If it's because of their work, the word "due to" could be used: Sometimes my parents aren't at home due to their work and so I have to stay at home alone with my brother.
2) I felt sorry for my mum because she has a broken leg (better: she broke her leg, she had a broken leg).
The change suggested is indeed better. Instead of "has," you could use "she broke her leg" or "she had a broken leg" to indicate that the leg-breaking incident occurred in the past and is not ongoing.
3) I finished primary school with an average mark of 9 out of (?) 10.
To determine what belongs in the blank, you will need to know the maximum possible score. If the maximum score is 10, then you could simply say: I finished primary school with an average mark of 9 out of 10.
4) I was proud of myself on 24th July when I passed my driving test.
This sentence is already correct and doesn't require any changes.
5) The teacher reprimanded me (= wrote a reprimand in the class register??)
To clarify what happened when the teacher reprimanded you, you could include more details. For example: The teacher reprimanded me and wrote a note of reprimand in the class register.
6) It was the last sad moment in my life when my girlfriend broke up with me (?). While I was climbing a step I fell on the floor (?)(I fell down on the floor)
The additional question marks may not be necessary in this case, as the sentence seems complete without them. However, to clarify the actions better, you could rephrase the sentence as follows: It was the last sad moment in my life when my girlfriend broke up with me. As I was climbing a step, I fell down on the floor.