Each of the reports contains useful information.
The subject of a sentence is the noun that performs the action.
If the sentence has a linking verb, then the subject is the thing that is being described. If you have a verb, that is the word that describes the action that the subject takes.
Sometimes, if you have several different nouns and verbs in a sentence, identifying the correct subject and verb can be difficult. It depends on the meaning of the sentence. For this one, though, the categorization is quite simple.
- Subject: Each of the reports (specifically, every report)
- Verb: contains
