1. Answer: The key reason of the problem is that these two verbs are different kinds of verbs. Keep is a copulative verb followed by an adjective, which often forms the table structure of the main system. Maintain is a notional verb and a transitive verb, which must be followed by a noun, thus forming a subject-predicate-object structure.

2. Grammar: The copula is a verb form different from the notional verb. Connective verbs, also known as connective verbs, are used together with predicates to explain the status, nature and characteristics of the subject. The verb function of copula is weak, so it must be combined with adjectives, nouns, gerunds, infinitives, prepositional phrases and quantitative phrases to express semantics. In English, there are mainly the following categories as copula verbs: ① The copula verb be that expresses the judgment or description of the state. (2) The copulative verb indicating development and change. Like becoming, going, getting, growing. (3) The copula indicates maintaining a certain state, such as maintaining, staying and maintaining. (4) Verbs expressing sensory feelings, such as feeling, smell, sound and taste.

3. Example:

It's actually very cold in the north. Winter in the north is usually very cold.

It has become more and more popular for young people to keep healthy. Fitness is becoming more and more popular among young people.

You can keep healthy and strong. If you exercise regularly, you can keep healthy and strong figure.

This pizza tastes so delicious. This pizza tastes delicious!