I understood well, but when I wrote, i made a mistakes. I understood well, but when I wrote, i made a mistakes.

I understood it well, but when I wrote, I made mistakes. (I understood the lesson, I understood it)

I understood the lesson well, but when I wrotepracticed writing, iI made asome mistakes. I understood the lesson well, but when I practiced writing, I made some mistakes.

a task is very difficult to me. a task is very difficult to me.

This task is vey difficult for me. OR I find this task very difficult.

aI felt that the task iwas very difficult to me. I felt that the task was very difficult.