I heard that he had worked for the FBI

[keannu]

My grammar book says that if it's evident that the subordinate clause's "tense" is prior to the main clause's "tense"(had worked < heard), you can only use a simple past tense like "worked". Is it because "before" makes "worked" one tense prior to "heard"? Does it depend on context or isn't this rule strict?

gz175)I heard that he had worked for the FBI.
= I heard that he worked for the FBI before.

 

[5jj]

There is no such rule.