Explanation
Object questions ask about the thing or person that the verb affects.
In "What did you buy?", "what" asks about the thing, "did" is the helper for the past tense, "you" is the subject, and "buy" stays in the base form. English puts the question word at the front. Japanese often says "何を買いましたか" and keeps the object before the verb. So "What you bought?" and "What did you bought?" are both wrong.
目的語をたずねる疑問文は、動詞の対象になる「もの」や「人」を聞く文です。
たとえば "What did you buy?" では、"what" が「何を」、"did" が過去の疑問を作る助動詞、"you" が主語、"buy" が動詞の原形です。英語では疑問詞を文頭に置きます。日本語は「何を買いましたか」のように、目的語を動詞の前に置きます。だから "What you bought?" や "What did you bought?" は不自然です。
Examples
- What do you want? 何がほしいですか。
- Who does she call after work? 彼女は仕事のあと、だれに電話しますか。
- What did you buy at the station? 駅で何を買いましたか。
- Which book did you choose? どの本を選びましたか。
- What are you looking for? 何を探していますか。
- What didn't you tell me? 私に何を話してくれなかったのですか。
Common Mistakes
Japanese learners often keep Japanese word order and leave out the helper verb: "What you want?" -> "What do you want?" They also use the past form after "did": "What did you bought?" -> "What did you buy?" After "did", use the base verb.
日本語の語順のままにして、助動詞を入れないミスがよくあります。"What you want?" → "What do you want?" です。また、"did" のあとに過去形を使ってしまうミスも多いです。"What did you bought?" → "What did you buy?" です。"did" のあとでは動詞は原形にします。