Welcome back, Chuncan Feng! It's good to see you again. I hope you haven't been finding the new platform too disorienting. The Search function is located in the toolbar at the top. Click on the magnifying-glass icon and you will have the option to use Search and Advanced Search. I generally use Advanced Search.
The following sentence is the key to a Chinese-English translation exercise:
(1) The biggest change in my life was that I am busier than before.
I think "is" is better than "was" here because the sentence is about a current change as suggested in the that-clause. Am I right?
I agree with you that "is" is better. "Was" sounds rather awkward to me. If I used the past tense ("was") there, I would change "am" to "became" and delete "than before": "The biggest change in my life was that I became busier." Informally, you could also use "got": "The biggest change in my life was that I got busier."