どこ

どこ asks "where". It is a place word: it sits where the answer would sit, and the sentence ends in か. With です it needs nothing else.

It is one of a set of four:

• ここ – here, where the speaker is
• そこ – there, where the listener is
• あそこ – over there, away from both
• どこ – where

The answer puts a place where どこ was: といれトイレはあそこです.

With particles

Before a verb, どこ takes the particle the answer would take: に or へfor a destination, で for the place something is done, に with あります and います, から for "from".

The particle follows the verb, not the English. "Where did you buy it?" is どこでかいました買いましたか, because buying is something done in a place. ×どこに買いましたか is wrong.

どこ and は

In a question, どこ does not take は. The thing you are asking about takes は and comes first; どこ comes after it. ×どこはトイレですか is wrong.

どこ can take が, when the place itself is the subject.

Usage notes

These go beyond asking where something is.

どこの

どこの before a noun asks where something comes from or which place it belongs to: which country, which company, which school.

Casual speech

Between friends です and か drop, and the voice rises.

どこか, どこも, どこでも

• どこか – somewhere
  しゅうまつ週末はどこかにいきたいです行きたいです。

  I want to go somewhere at the weekend.
• どこも with a negative – nowhere. A particle goes in the middle: どこにも, どこへも.
  きのうはどこにもいきませんでした行きませんでした。

  I didn't go anywhere yesterday.
• どこでも – anywhere at all
  どこでもいいです。

  Anywhere is fine.