Question 4 might be tricky if you're only working with the information presented by 7Sage. The sentence:

"The formation of hurricanes that threaten the United States mainland is triggered by high atmospheric winds off the western coast of Africa."

is in the passive voice, which means the subject is receiving the action. Not every passive voice sentence includes the thing performing the action that the subject is receiving, but when it does, that noun or noun phrase is called the agent. Structurally, it feels like an object, but an agent is its own thing, so 7Sage considers that just another modifier.

Let's try another example. Let me know what you think, and point out the agent if you're feeling bold.

The seminal work of proto-existentialist fiction Notes from Underground was written by Fyodor Dostoyevsky.

@JacksonHolt In this case, "milk" is an object, which is part of the predicate. It doesn't directly modify anything, so much as "answer a question" indirectly posed by the verb (either receiving or being affected by the action).

In your example, "the cat like(s) to drink milk," The subject and verb together ("the cat likes to drink") form a complete sentence, but it really only makes sense because we understand "likes to drink" figuratively as "likes to drink alcohol." We may be concerned for the cat's mental health and wellbeing.

Taken literally, "the cat likes to drink" needs to answer a question to get its barebones point across. What does the cat like to drink? Milk.

If you can answer a question about the basic subject-verb phrase in that way with a noun (or noun phrase) in the sentence, it's an object.

@KeziaH19 Absolutely. It's like music; practice hitting the right notes before you worry about tempo.

@Lemont.Williamson It's from a 1996 neo-noir movie called "The Long Kiss Goodnight."