FLOWER OF INTELLIGENCE

Plankton finally got the recipe. He had been trying to steal that for two years.

(+) S + had + been + v-ing + O + adverb of time
(-) S + had + not + been + v-ing + O + adverb of time
(?) had + S + been + v-ing + O + adverb of time.

Usage
To emphasize the duration of an activity that was in progress before another activity or time in the past.
Example:
The police had been looking for the criminal for two years before they caught him.
I had been studying Arabian language for two years last month.

Frequently used adverb
For …, by yesterday, since …, by last ….


Note:
All subject use ‘had + been’.