I just thought to add some insight. In my opinion,
1."During my studies, I have worked on different projects" means your studies began in the past and are still continuing now, in present time.
2."During my studies, I worked on different projects" means your studies began AND ENDED in the past, so there �s no connection to present time.
(Therefore, both #1 and #2 are grammatically correct. You have to choose based on the information you are trying to convey.)
3. "I have worked as foreman assistant during my last training period" doesn �t make sense because your last training period ended in the past, so present perfect "have worked" doesn �t fit. (It would make more sense to say instead: I have worked as foreman assistant recently.)
4. "I worked as foreman assistant during my last training period." makes sense because your last training period was in the past and the verb "worked" is in the past tense. The grammar is parallel, so it is correct.
Hope this helps!
