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?

[2010]

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?

===Not a teacher===

I would prefer using "writing" than "doing" (tough will remain the same)

If you want to refer something that happened in the past, you could say...

I had a tough time writing those\the exams.("these" can be used if you are talking about the subject having in front of you to show it to your friend)