When do we use" Doing"

[englishquestion]

When do we use" Doing" ..normally we use it to indicate that we are doing something but in following sentence I find it different Please check this and elaborate on the usage of doing :
1) I had a tough time doing these/the exam (Can we also use writing instead of tough in 1st and 2nd sentence)
2) I was doing my exams when you called
Also is the follwoing sentene if my exams are already over last week and if I am saying this sentence after that to my friend then should I say "These" or "The" exams and why?

 

[emsr2d2]

When do we use" Doing" ..normally we use it to indicate that we are doing something but in following sentence I find it different Please check this and elaborate on the usage of doing :
1) I had a tough time doing these/the exam (Can we also use writing instead of tough in 1st and 2nd sentence) - I think you meant to ask "Can we also use "writing" instead of "doing", not "writing" instead of "tough"!)
2) I was doing my exams when you called.
Also is the follwoing sentene if my exams are already over last week and if I am saying this sentence after that to my friend then should I say "These" or "The" exams and why?

We don't usually "write" an exam. We do an exam. We take an exam. We sit an exam.

If the exams are over, I would say "I had a tough time doing those exams". Those = the specific exams which you took last week. You can also say "I found those exams really tough".