不定詞（to + 動詞の原形）The infinitive (to + base verb)

基本の説明Overview

不定詞は〈to + 動詞の原形〉の形です。この形が、文の中で名詞・形容詞・副詞のような働きをします。「〜すること」「〜するための」「〜するために」のように、場面によって日本語の訳し方が変わります。The infinitive is the form “to + base verb.” It works like a noun, an adjective or an adverb inside a sentence, so the Japanese translation changes depending on the use: “to do,” “to do (for it)” or “in order to do.”

まずは次の3つの用法を整理しましょう。どの用法でも、to のあとの動詞は主語や時制に関係なく、いつも原形です（She wants to go. のように -s はつけません）。First, let's sort out the three uses below. In every use, the verb after to is always the base form, whatever the subject or tense (She wants to go. — no -s).

名詞的用法は、want to〜（〜したい）、like to〜（〜するのが好き）、hope to〜、decide to〜、begin to〜 のように、動詞のあとに続けて使うことが多く、It is ... to〜 の形や、疑問詞 + to〜（what to do など）もこの仲間です。The noun use often follows verbs such as want to ~, like to ~, hope to ~, decide to ~ and begin to ~. The pattern It is ... to ~ and question word + to ~ (what to do, etc.) belong here too.

例文で確認Example sentences

① 名詞的用法（〜すること）1. Noun use (“to do”)

② 形容詞的用法（〜するための／〜すべき）2. Adjective use (“to do (for it)”)

③ 副詞的用法（目的：〜するために）3. Adverb use (purpose: “in order to”)

④ 副詞的用法（感情の原因：〜して）4. Adverb use (reason for a feeling: “to do”)

⑤ It is ... to〜 と、疑問詞 + to〜5. It is ... to ~ and question word + to ~

確認クイズQuick check

カッコの中の語を、文に合う形にして空所に入れましょう（最後の問題は用法を答えます）。Change the word in the parentheses into the form that fits and write it in the blank (the last question asks for the use).

1. I want ___ a new bike. (buy)
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   to buy — want のあとは〈to + 動詞の原形〉。want is followed by to + base verb.
2. I have a lot of things ___ today. (do)
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   to do — things を後ろから説明する形容詞的用法。「やるべきこと」。The adjective use explains things from behind: “things to do.”
3. I went to the library ___ some books. (borrow)
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   to borrow — 「借りるために」という目的を表す副詞的用法。The adverb use of purpose: “in order to borrow.”
4. We were sad ___ the news. (hear)
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   to hear — 感情の原因を表す副詞的用法。「知らせを聞いて悲しかった」。The adverb use for the cause of a feeling: “sad to hear the news.”
5. Please tell me how ___ this camera. (use)
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   to use — 「how + to + 動詞の原形」で「使い方」。how + to + base verb means “how to use.”
6. I was surprised to see him. の to see は、名詞的・形容詞的・副詞的のどの用法？
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   副詞的用法Adverb use — 「驚いた」という感情の原因を表している。It gives the cause of the feeling “surprised.”

おさえておきたいポイントKey points

• 不定詞は〈to + 動詞の原形〉。to のあとの動詞は、主語や時制に関係なくいつも原形The infinitive is to + base verb; the verb after to is always the base form, whatever the subject or tense
• 3つの用法：名詞的「〜すること」、形容詞的「〜するための」、副詞的「〜するために／〜して」Three uses: noun “to do,” adjective “to do (for it),” adverb “in order to do / to do”
• 名詞的用法は want / like / hope / decide / begin / try / need などのあとでよく使うThe noun use often follows want, like, hope, decide, begin, try, need and similar verbs
• 形容詞的用法は、名詞や something などの代名詞のあとに置いて「〜するための」と説明するThe adjective use comes right after a noun or a pronoun like something and explains it
• 副詞的用法は、目的（〜するために）と、glad / happy / sad / surprised などの感情の原因（〜して）で使うThe adverb use shows purpose (“in order to”) or the cause of a feeling after words like glad, happy, sad or surprised
• It is ... (for 人) to〜 や、what / how + to〜 の形も覚えておくAlso learn It is ... (for someone) to ~ and what / how + to ~