<p>Today the primary role of advertising may be to appeal and persuade rather than what it once did, educating and informing.</p>
<p>so the answer is: what it once was, to educate and inform.</p>
<p>my question is: why is it “what it once was” and not “what it once did”</p>
<p>I’m not totally sure, but it seems to me that in the phrase “role of advertising may be to appeal and persuade”, “to appeal and persuade” is an infinitive phrase functioning as an predicate noun, basically renaming or defining “roll”.</p>
<p>This means that the next part “rather than what it once did, educating and informing.” must be in the form “rather than what it once was, to educate and inform,” in which case, “to educate and inform” also functions as an infinitive phrase renaming “it” which is a pronoun for “role”. Basically, you have to identify how the different phrases are functioning in the sentence and then make sure it is parallel throughout.</p>
<p>That’s because the subject of was/did in the underlined part is “role” and not “advertisement”. (“it” is a pronoun for “role”)</p>