I accomplfinished a preparatory class, but iI don’t speak andor understand English, so iI decided to use this website forto improvinge my writing skills. I finished a preparatory class, but I don’t speak or understand English, so I decided to use this website to improve my writing skills.

I accomplispassed thed preparatory class, but iI don’t speak andor understand English, so iI decided to use this website for improving my writing skills. I passed the preparatory class, but I don’t speak or understand English, so I decided to use this website for improving my writing skills.

I accomplishedpassed a preparatory class but iI don’t speak andor understand English, so iI decided to use this website forto improvinge my writing skills. I passed a preparatory class but I don’t speak or understand English, so I decided to use this website to improve my writing skills.

"for improving" versus "to improve" both are correct but to improve sounds more natural speaking

I want to improve my English because i’I'm studying chemical engineering in English. I want to improve my English because I'm studying chemical engineering in English.

I want to improve my English because iI’m studying chemical engineering in English. I want to improve my English because I’m studying chemical engineering in English.

"I" pronouns are always capitalized.

I want to improve my English because i’I'm studying chemical engineering in English. I want to improve my English because I'm studying chemical engineering in English.