I have left the correct forms, Hela: Dear Bob, I AM JUST WRITING to let you know our new address and to invite you TO our house-warming party next Saturday. I AM sorry ABOUT/FOR the lack of warning, but we HAVE BEEN / WERE busy moving house and I HAVE HAD LITTLE time for anything else. IN any case we ONLY DECIDED to hold the party last week when we FOUND OUT that the cost of moving WAS not as high as we HAD RECKONED / RECKONED and that we HAD A LITTLE / SOME cash to spare.
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The trouble WAS (can we put “IS” here ?yes) it 3) arrived 4) in the middle of the summer exams and I 5) was just too busy to write back.
CJ: The reason is that he likes to stay up late at night.
CJ: The reason is/was that he stayed up late the previous night.
[No, I think 'is' is fine. You can almost always state what a reason is to focus on your current opinion of what that reason is.