Sometimes, all you need is a quick, merciless judgment.
A Sparkler writes,
Is this sentence grammatically correct?
"Lydgate idealizes Rosamund, he didn't figure out how materialistic she is"
The short answer: no.
The long answer: no, for four reasons.
1) Tense shifts. It's okay to use different tenses in the same sentence, as long as you're obeying grammar rules. Here, you're shifting tenses in an illegal fashion. You begin in present ("idealizes"), move to past ("didn't), and go back to present ("is") in a way that doesn't make sense.
2) Comma splice. You've got two independent clauses here, which means you can't connect them with a comma.
3) Sense. It took me a few reads to understand the idea you're trying to convey. That's mostly a matter of style, but the grammar problems also obscure meaning.
4) Punctuation. You're missing a period. No big deal, but teachers tend to get grumpy when you leave out punctuation.
Here are a few ways you might revise this sentence:
Lydgate idealizes Rosamund because he doesn't realize how materialistic she is.
Lydgate idealizes Rosamund; he does not grasp how materialistic she is.
Lydgate idealizes Rosamund, refusing to see how materialistic she is. (I think this rewrite expresses the idea most clearly.)
Want me to check your sentence? Send it to firstname.lastname@example.org, and you might see your grammar corrected on the blog!