This is how i learned it in 6th grade:

any prepositional phrase in the sentence cannot be apart of the object, noun or verb. while they are important to the sentence, they are not important in the structure.

eg: "to provide adequate facilities", "for physical education", "for the grant" these are disqualified from being the subject, verb, and object in the first sentence bc they are in prepositional phrases

how you know when to cut the prepositional phrase off: if the verb/preposition are answered or another preposition is presented

hope this is helpful!