I would like to know how to express mandatory conditions in Japanese. Since Japanese distinguishes between different types of languages, I should probably ask more precisely:

What is the right way to express equivalence statements (the kind a mathematician might make) such as "The equation is satisfied if and only if a>b"?

How do you express that you're going to do or that something will happen, only if some condition is satisfied? ("I'll only go if you buy me an ice.", "The plant will only grow if it is watered regularly.")

  • The last paragraph (usual conditionals) is quite far removed from the question about the technical language ("if and only if"). There are several ways of expressing conditionals, and we have many questions about them (click on the [conditionals] tag to find them). I suggest you focus your question on the first part.
    – Earthliŋ
    Feb 15, 2016 at 18:55
  • I'm not confident enough about this as an answer, but I think that pattern is ~さえ~ば. 植物に水さえやれ成長する. This pattern implies "if", but not sure about the "only if".
    – istrasci
    Feb 15, 2016 at 19:02
  • 2
    @istrasci It'd be "if only", right? Feb 15, 2016 at 19:20

2 Answers 2


Two common ways of translating "if and only if" use the terms 必要十分条件 ("necessary and sufficient condition") and 同値 ("equivalence").

a > b は式 (15) である為の必要十分条件である。
Equation (15) holds if and only if a > b.

式 (15) と「a>b」とは同値である。
Equation (15) is equivalent to a > b.

  • 否{ひ}定{てい}排{はい}他{た}的{てき}論{ろん}理{り}和{わ} if you want to write your logic in 漢文 :) This is the full name for an XNOR logic gate, which has the same operation as an iff statement.
    – a20
    Jun 1, 2020 at 16:44
  • if and only if (= iff)

    a > b の時、(そして/かつ)その時に限り等式が成立する。
    The equation is satisfied if and only if a > b.

  • only ... if

    • ~なければ~ない (colloquially ~なきゃ~ない or ~なけりゃ~ない)
      ≈ ~ないなら~ない

      I'll only go if you buy me an ice.

    • ~ないと~ない

      The plant will only grow if it is watered regularly.

    • The difference between them is, in short, you can't use the latter if your(= the speaker's) will intervenes.

  • I thought that the technical translation usually involves 必要十分(条件) (or sometimes 同値 "equivalent").
    – Earthliŋ
    Feb 15, 2016 at 19:13
  • @Earthliŋ Yes, AとBは同値 would be shorter, but you can't put it into this word order. This expression is a mouthful, yet fixed. Feb 15, 2016 at 19:16

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