|Plankton finally got the recipe. He had been trying to steal that for two years.|
(-) S + had + not + been + v-ing + O + adverb of time
(?) had + S + been + v-ing + O + adverb of time.
To emphasize the duration of an activity that was in progress before another activity or time in the past.
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 ….
All subject use ‘had + been’.