Take a look at the sentence again:
If Tyree had __________ how beautiful the substitute teacher was going to
be, he would have prepared his math homework—instead of an arsenal of spitballs—to
impress her with his intelligence.
The sentence requires the past participle of know, an irregular verb. Known is the past participle form.
| Verb | Simple Present | Simple Past | Past Participle |
| know | know(s) | knew | known |
Knew, the form you chose, is simple past. You cannot use simple past if you have an auxiliary verb like had. Before you continue, review the rules for irregular verbs.
Try again to escape Grammar Hell!