I know it's a very basic question, and I've looked up the definition of "subject." This very question was asked within a linguistic course that I am taking, the answer to which is apparently rice (see the example sentence below). I am quite surprised by this answer, for I thought that the subject is that which acts, and the object is that which is acted upon. So the sentence 'Rice is being cooked by Mary', could always be restated as 'Mary is cooking rice', and surely here, Mary is the subject. Anyway, the reason provided as to why rice is apparently the subject is as follows:
"'Rice is the subject of this sentence. When you change the word rice into 'potatoes' the verb changes, but if you change 'Mary' into 'Mary and John', the verb does not change."
(a) Potatoes are being cooked by Mary
(b) Rice is being cooked by Mary and John
(Emphasis not my own)
I am also having a hard time understanding the connection between the significance of a change or lack thereof of the verb, and the identification of the subject within the given sentence.