Please look at this sentence.

"The young and aggressive dogs are kept separate from the aged and weak ones"

[Q] Is this sentence right in grammar? I think he should have written like this.
"The young and aggressive dogs are kept separately from the aged and weak ones"
Because I think the words "the young and aggressive dogs are kept" is a perfect sentence(a clause) itself. So its next word should be one among to infinitive,gerund,adverb,adverbial,preposition phrase or so on. How can it directly bring an adjective "separate"?
Am I wrong?

Thank you for your answer in advance.
That's a good question.

That sentence is correct and natural. The important phrase is kept separate from. If you keep something separate from something else, you are deliberately making sure they cannot come into contact with each other. There is an intention to maintain separation between them.

Your suggestion, that aggressive dogs are kept separately from the weak ones, is also valid, but the meaning of kept is subtly different. In your sentence, kept simply refers to the dogs being housed; in the original sentence, kept is part of the phrase kept separate, where keep means maintain or ensure, as in "keep your eyes on the road".

Of course, keeping (maintaining) these two groups of dogs separate from each other would involve keeping (housing) them separately, i.e. in separate cells or rooms or areas, but that is not the essential point of the sentence; the emphasis is on preventing the groups from being together by keeping them separate from each other.
Do you mean that I should treat "kept separate from" as "an adjective phrase"? Oh! It's so difficult to me. Thank you so much for your kind explanation,Mr.KrisBlueNZEmotion: embarrassed
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I don't think it's an adjective phrase. Keeping some dogs separate from others is an activity. If you rephrase that sentence as "We keep the young dogs separate from the old dogs" you can see that kept and kept separate from are used as some kind of verb.

I'm afraid I don't know enough about English grammar rules and terminology to be able to explain it any better than that. Perhaps if you have an English teacher you could ask them. And when you find out, let me know as well, ok? :-)
Ah~ I've already read the first answer of yours more than three times and now I feel if I read your answers more, I can understand it.I've found another sentence in my dictionary.

"Raw meat must be kept separate from cooked meat". Perhaps I think I need to memorize this sentence from as a particular case. I will read your answers again tomorrow. Thank you so much for your kind answers,Mr.KrisBlueNZEmotion: embarrassed.
I think I've found it.

eg1)I make you happy

-> You are made happy by me.

eg2)I make you waste money

-> You are made to waste money by me.

Causative verbs like make,have,keep,let ! When they are in the passive voice, as per adjectives, we don't need to change.

but as per verbs (eg2), we need to change to "to infinitive". I've read your answers again and again.Thanks to you, I think I've found it. Thank you so much for your explanation all,Mr.KrisBlueNZEmotion: embarrassed.
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You're welcome :-)

Right, "raw meat must be kept separate from cooked meat" is a perfect example.

I think you're right with your explanation. Thanks for explaining it to me! I will make a note of it.