Thank you very much, Writeacher. I need to revise these sentences, too.

1) I was last proud of myself when I helped one of my friends one week ago.
She lost her father after a tragic accident (better: in) and I'm currently supporting her emotionally.
2) I often blamed (correction blame) a schoolmate for acting in a childish way (childish).
3) I'm still dependent on my mother but just for the money because I don't have a job.
4) I felt sorry for a friend because we saw her father dying while we were travelling on the bus. The man fell off a 30m-high cliff while he was driving his car. He had a heart attack.
5) I sometimes feel ashamed but I don't care that much if it is not a serious problem and I enjoy myself.

Shall I evaluate it a mistake if someone keeps wriing "i" instead I when the personal pronoun occurs whithin a sentence?

1) I was last proud of myself when I helped one of my friends one week ago.

She lost her father after a tragic accident (better: in) and I'm currently supporting her emotionally.
You'd use "after" if there was an accident and he died later (in the hospital or at home). You'd use "in" if he was killed during the accident itself.

2) OK

3) OK

4) OK

5) I sometimes feel ashamed but I don't care that much if it is not a serious problem and I enjoy myself.
Three main clauses? This sentence needs to be divided into two.

Shall I evaluate it a mistake if someone keeps wriing "i" instead I when the personal pronoun occurs whithin a sentence? Yes. Every time!