Help for my CELTA, please?

May I ask an English teacher, or anybody with knowledge regarding the following questions for some assistance on a few select questions that I am having some difficulty with on my CELTA Pre-Interview Task? I'm trying to get into CELTA school in Toronto. It's kind of urgent because the slot I'm trying to enroll in is starting to get full, so I received a call from one of the administrators to tell me to hurry up if I want to be in class by May 31st or else I'm going to have to wait till July. If this is an inappropriate location to make this post, I'd like to request for the moderator to relocate it to its proper area since I don't have authorized access to the other forums. If anybody can help it would be greatly appreciated and if not, I'll just go with what I think I know which I presume to be, well, sort of incorrect.

How would you explain to a language learner the difference in meaning between the following pairs of
sentences:
a) When he arrived, we had eaten dinner.
b) When he arrived, we ate dinner.
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c) By this time next year, I will have finished college.
d) By this time next year, I will be finishing college.
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e) I've been busy all morning, I've painted the porch.
f) I've been busy all morning, I've been painting the porch.