Root phrase in this example/why can you use "it"?

[yo4561]

Happy New Year!

I received the following example from an instructor:
"I ate pizza yesterday and will eat it tomorrow. I ate + I will eat.
To test whether the elements are parallel, skip from the root phrase to each element and check for clarity. This is why the sentence in the previous bullet works: I ate pizza yesterday; I will eat it tomorrow."

To clarify, is the root phrase just "I"?
Also for parallelism, why don't you need to repeat pizza in place of "it" in the phrase "will eat it"? Is it because this is still one sentence, so you know what "it" is referring to? I am just a little confused on if you can use the antecedent in the "X" portion and the pronoun in the "Y" portion (assuming X and Y structure)?

Many thanks MP

[esledge]

To clarify, is the root phrase just "I"?
Also for parallelism, why don't you need to repeat pizza in place of "it" in the phrase "will eat it"? Is it because this is still one sentence, so you know what "it" is referring to? I am just a little confused on if you can use the antecedent in the "X" portion and the pronoun in the "Y" portion (assuming X and Y structure)?

Yes, the root phrase is just "I" in those examples.

There are a few reasons you don't need to repeat "pizza":
(1) There is nothing else for "it" to refer to in the sentence, so the antecedent of the pronoun is clear.
(2) Because "it" clearly refers to "pizza," you actually DO have perfect parallelism between the objects of the verbs: I (ate/will eat) the same food on both days!
(3) Parallelism rules don't require you to have the same object (pizza), anyway: If "I" is the root phrase, the parallel elements just have to (a) be grammatically similar to one another and (b) both be logical predicates for "I."

For example: Lu ate pizza yesterday and will visit the Colosseum tomorrow.

Lu (who might be on vacation in Rome) is the subject of both ate pizza and will visit the Colosseum have . The verbs don't have to be the same tense, and the objects don't have to be the same things (because you can't eat the Colosseum and it's weird to visit pizza)!

[esledge]

I am just a little confused on if you can use the antecedent in the "X" portion and the pronoun in the "Y" portion (assuming X and Y structure)?

Oh, and I just remembered the second part of your question. Yes, you can do this--in fact, it is very, very common. Indeed, the parallel structure often helps identify the pronoun/antecedent relationship, even in some sentences where the pronoun might otherwise be ambiguous!