Learn English – Data is “stored in a memory” or data is “being stored in a memory”

I thought that the subject is that which acts, and the object is that which is acted upon.

This is often true in an active-voice sentence, but not in a passive-voice sentence.

That which acts/is acted upon and subject/object really describe two different categories, not a single category.

• That which acts and that which is acted upon are semantic roles, the roles a word or phrase plays with respect to the meaning of an utterance. The usual terms in linguistics are Agent (actor) and Patient (acted upon).

• Subject and Object are syntactic roles, the roles a word or phrase plays with respect to the structure of an utterance.

In an active-voice sentence the Subject is also the Agent^†, and if the verb is transitive the Patient is the Direct Object. (If the verb is intransitive there is neither an obect nor a patient.)

In a passive-voice sentence the Subject is the Patient; the Agent may be omitted or expressed as an Oblique, the complement of a preposition phrase with by.

A syntactic rule in English is that in a clause headed by a finite verb that verb must 'agree' with its Subject in person and number—that is, the number and person of the Subject contribute to determining what inflection the verb bears. The only distinctive inflections affected by this rule in contemporary English are the 3d person singular -s inflection with regular verbs, the have/has constrasts with HAVE, and the am/is/are and was/were contrasts with BE.

In the two sentences cited in your example, one sentence uses rice and is and the other uses potatoes and are. Otherwise the two sentences are identical. Rice is singular and potatoes is plural; that is the only difference which can account for the is/are contrast, so rice and potatoes must be the subjects of their respective sentences.

^† _{Actually, many sentences, such as those headed by "linking" verbs like be, become, seem, have no Agent or Patient—they present a different set of semantic roles—but that's not something we need to get into here.}

The first sentence is not correct: you should not use passive voice for start in this context, because the main activity is turning, not starting. It is optional for turn:

The steering wheel emitted a strange sound when it started to turn.
The steering wheel emitted a strange sound when it started turning.
The steering wheel emitted a strange sound when it started to be turned.

The first two versions might give the impression that the wheel turned on its own: the second version sounds over-complex.

It would make a more natural sentence if you kept the steering wheel as the subject of the first clause, but made it the object of the second clause:

The steering wheel emitted a strange sound when I started to turn it.

emit is a bit technical: make would work better in informal spoken English.