Can चिट्ठी also mean "note"? (As in "The teacher didn't see as my note to Jenny passed through the classroom.")
Is my understanding correct that we don't know the gender of उसने here? Since the verb पढ़ी takes the gender of चिट्ठी? In other words, would "He read your letter" also be correct?
“He read your letter” would be उसने तुम्हारी चिट्ठी पढ़ा, I think – that is, पढ़ा rather than पढ़ी.
No. slacker822 is right. Since पढ़ना is a transitive verb, it takes the gender of its object in the simple past tense (चिट्ठी in this case which is feminine). So, 'He read your letter' would also be 'उसने तुम्हारी चिट्ठी पढ़ी'.