Step 1: Identify the tense of the original sentence.
The given sentence “I am doing my bit” is in Present Continuous tense. Structure: Subject + am/is/are + verb (ing).
Step 2: Recall the rule for Past Perfect Continuous.
Structure: Subject + had been + verb (ing) + object.
Step 3: Transform.
Replace “am doing” with “had been doing.”
Hence, the sentence becomes: “I had been doing my bit.”