Subject and object while using passive voice [closed]

My English teacher and an overwhelming majority of my English class insists that in the following sentences the bolded words are subjects and the italicized words are objects.

I ate the cake.

The cake was eaten by me.

The first sentence is obviously correct, but I'm fairly certain that in the second sentence "cake" is the subject and "eaten by me" is the object.

The cake was eaten by me.

May I have some concrete evidence proving or disproving my point?

In the first sentence, traditional grammar regards I as the subject and the cake as the object. In the second sentence, the cake is the subject and there is no object.

Functional grammar, however, takes a rather different view. It calls the subject and object Participants and the verb a Process. In the first sentence, I and the cake are Participants and ate is a Process. More specifically, I is the Actor, the cake is the Goal and ate is a Material Process. In the second sentence, me remains the Actor and the cake remains the Goal, even though their structural roles have changed.

Usually, the Subject is essentially regarded as being the element with which the verb agrees. In your passive example, "the cake" is the subject: change this to "the cakes" and you see that the verb changes from "was" to "were", whereas changing "me" to "us" or "them" makes no difference to the verb form.

There are a few exceptions and corner cases, notably a phenomenon whereby what you might regard as a 'structural' subject position actually gets filled with a "dummy" or "filler" subject (think about a sentences such as "There appeared before him a ghostly figure" or "Let there be light"). But by and large for your purposes here, you can regard the subject as the element that agrees with the verb.