The simple subject is the term used for the actual word that is the subject of the verb. The simple subject may have modifiers before or after it, but once these are removed, the word left is the simple subject.
The hungry fox darted across the yard.
("The" and "hungry" are modifiers for the simple subject "fox".)
The small carton of nails was found under the seat.
("The", "small" and "of nails" are modifiers for the simple subject "carton".)
The modifiers around your simple subject sometimes make it seem plural, even though it might be singular.
A container of nuts and bolts were found in the attic.
(should be was / The simple subject is "container".)
See also Prepositional phrases
Glossary of grammatical terms