When I studied in a technical secondary school, my major was agricultural machinery management. When I studied in a technical secondary school, my major was agricultural machinery management.

I studied agricultural machinery management when I was in a technical secondary school.

When I studiedwas studying in a technical secondary school, my major was agricultural machinery management. / My major when I was in technical secondary school was agricultural machinery management When I was studying in a technical secondary school, my major was agricultural machinery management. / My major when I was in technical secondary school was agricultural machinery management

"Was studying" since it happened over a period of time

I didn't studytake the course seriously because students in a technical secondary school back then were assignguaranteed a job by the government after they have graduated. I didn't take the course seriously because students in a technical secondary school back then were guaranteed a job by the government after they have graduated.

I didn't take my studyies seriously back then because students in a technical secondary school back then were assigned a job by the government after they graduated. I didn't take my studies seriously back then because students in a technical secondary school were assigned a job by the government after they graduated.

Not wrong, but I think it sounds better this way

After I graduated from that school, I was assigned a job as a teacher in an agricultural mechanization school of a county in another prefecture -level city. After I graduated from that school, I was assigned a job as a teacher in an agricultural mechanization school of a county in another prefecture-level city.

After I graduated from that schooling, I was assigned a job as a teacher in an agricultural mechanization school of a county in another prefecture level city. After graduating, I was assigned a job as a teacher in an agricultural mechanization school of a county in another prefecture level city.

Just "graduating" is fine

ThaIt meant that I was going to teach the tractor drivers in that county soon. It meant that I was going to teach the tractor drivers in that county soon.

That meant (that) I was going to teach the tractor drivers in that county soon. That meant (that) I was going to teach the tractor drivers in that county soon.

So I had to study my courses that I should have learned well when I wasrevise the course content that I should have originally mastered in school very hard before I started to teaching the drivers. So I had to revise the course content that I should have originally mastered in school before I started teaching the drivers.

So before I started teaching the drivers, I had to study mythe courses that content again, even though I should have learned them well when I was in school very hard before I started to teach the drivers. So before I started teaching the drivers, I had to study the course content again, even though I should have learned them well when I was in school

Your sentence is phrased a bit awkwardly

It took me about one or two months to studycover the course materials. It took me about one or two months to cover the course materials.

It took (me) about one or two months to study the courses. It took (me) about one or two months to study the courses.

When I taught my first class, I found out that all the drivers were older than me, which made me nervous. When I taught my first class, I found out that all the drivers were older than me, which made me nervous.

I even forgotnded up forgetting what I had prepared for the class, so I finishended the class very early and. What's more, right after the class ended, I couldn't even remember what I thohad taught in the class just after the class ended. I ended up forgetting what I had prepared for the class, so I ended the class very early. What's more, right after the class ended, I couldn't even remember what I had taught in class.

I even forgot what I prepared for the class, so I finished the class very early, and I couldn't even remember what I thought in thewas thinking during class just after ithe class ended. I even forgot what I prepared for the class, so I finished the class very early, and I couldn't even remember what I was thinking during class after it ended.

Yet now, I can evenstill clearly rememberpicture the scene of thate class now, becaussince it was the first class ofin my career. Yet now, I can still clearly picture the scene of the class since it was the first class in my career.