Adjectives, Adverbs, Present Continuous and Modals Of Obligation

The word 'nice' in the sentence above is an adjective

This sentence is written in Present Simple,
Thus, the correct answer is 'learn'

We DON'T need to add s/es after the verb for They/I/We/You

The sentence is written in Present Simple

We NEED to add s/es after the verb for She/He/It
Thus, the correct answer is 'plays'

The correct option to fill in the blank is 'have to'

Have to means that something is necessary to do because there's a rule or a command from other people, especially those who are in authority.

We use 'should' when we give a suggestion or an advice to someone.

E.g: You got a fever. You should go to the doctor now.

An adjective is a word that describes a person, an animal or a thing.

E.g: She has a long straight hair.

An adverb of manner is a word that describes how someone do something.

E.g: She speaks Spanish fluently.

The sentence above has 'best' as an adjective.