です after some verbs (ほしいです)

Question

I've learned that です means something like the state of being, or close to the "be" verb.

Recently I found the following phrase: 犬がほしいです。 Why does it end with です if I am saying that "I want a dog"? (the verb is "want").

Answer 1

ほしい isn't a verb, it's an adjective. You can think of it like the English word "wanted"or "desired". So, a ほしいもの is a wanted or desired thing.

Answer 2

Short answer:

It's a politeness marker.

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Long answer:

You can't think of Japanese in terms of English.

In the English sentence I want a dog, it's true that want is a verb. But in the Japanese sentence 犬{いぬ}がほしいです, the word ほしい is an adjective, not a verb. Even though you can express the same basic idea in both languages, the grammar to do so is quite different.

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Why do we use be with adjectives in English?

In Standard English, we need a verb like be with nouns and adjectives, because nouns and adjectives can't form predicates on their own:

1a. *She silly.　　　　　 　 ←　ungrammatical
1b. She is silly.　　　　　　←　OK

2a. *She a programmer. 　 ←　ungrammatical
2b. She is a programmer.　←　OK

We need a verb like be. Why? It shows tense and subject–verb agreement, something nouns and adjectives can't do. The basic meaning comes from the noun or adjective, but we need be there for grammatical reasons.

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Adjectives can form predicates in Japanese

In Japanese, there's no need for a verb like be to show tense on adjectives. Adjectives can indicate tense all on their own, using the endings -い and -かった:

3a. 美{うつく}しい　　　　　'is beautiful'
3b. 美{うつく}しかった　　　'was beautiful'

Each of these is a complete predicate on its own without です. However, as you've noticed, people sometimes do add です:

4a. 美{うつく}しいです　　　'is beautiful (polite)'
4b. 美{うつく}しかったです　'was beautiful (polite)'

In this case, です doesn't have its usual grammatical function. Instead, it's functioning as a politeness marker. Note that tense is marked on the adjective with -い or -かった, so です doesn't change form to でした in 4b.

In other words, the is or was meaning is carried by the -い or -かった ending on the adjective. The です part serves only to make the predicate polite.