"Did you know I didn't realize it until I started this paper that the poem is often quoted?"
Here's why:
* "Did you know" is a question, so it needs a question mark at the end.
* "I didn't realize it" is a statement within the question, so it needs a comma after "it".
* "until I started this paper" is a dependent clause that explains when the realization occurred, so it needs a comma after "paper".
* "that the poem is often quoted" is a dependent clause that explains what the realization is, so it needs a comma after "quoted".
* "The poem" needs to be capitalized since it's a proper noun.
* "is often quoted" is a verb phrase that needs to be in the present tense to match the context of the sentence.
Let me know if you'd like to practice punctuating other sentences!