In one of the renowned grammar books I've come across an example of confusing usage of the perfect infinitive, where in my opinion past perfect should be used instead:
He is thought to have been deeply depressed at the time, but recovered later.
The above sentence is placed in the past, so why there is perfect infinitive? I'd say the following sentence is more logical:
He is thought to had been deeply depressed at that time, but recovered later.
Can anybody explain it to me, please?
You can't have a conjugated form after "to". "Had" has to have a subject in the third person (he, she, it, this man, a person, etc.); the subject of "is" is "he"; then there is no subject for "had"; that is because "had" is wrong here, you must have the bare infinitive: "have".
You also have to keep in mind the usual construction in the passive for "think".
Is only conjugated the first occurrence of "to be". When you state a fact about events, situations, people and so on and that this fact is still valid in the present , the plain infinitive is used.
If the fact is a thing of the past, then you use the perfect infinitive.
That is the case of your sentence: he was depressed in the past (at least some people think he was); it is a thing of the past.
