It contains many types of plants? what it the simple subject?

When you don’t understand a concept during the lecture, it’s to flag it in your notes and leave a blank space so you can fill in more

Which of these can an adverbial clause modify?

Why is it important to include a descriptive subject heading? it in sorting the topics. it the recipient see its importance. e-mail programs won’t allow e-mail to be sent without it. it keeps junk e-mail separate from important e-mail.