A preposition links nouns etc to the rest of the sentence, it often describes spatial positioning, like-
'She held her hand OVER the candle'- thus her hand is located in the sentence.
Also worked out the preposition...getting my brain back in gear..my grandaughter is doing her homework and keeps asking me these things.
Don`t they teach them anything in schools these day!!!!
A subject in a sentence does something for example: my sister works ..... If my sister is replaced by 'she' this is a pronoun subject.
A prepostion is usually before a noun and states position, when, how etc for example He stood under the rain, he came before breakfast, he arrived by car ....