An anonymous referee of a paper I wrote requires me to change all sentences of the form "If X, Y" to "If X, then Y". The paper is on theoretical computer science and contains many sentences like "If P is a triple, Gamma(P) equals zero." He requires me to change that into "If P is a triple, then Gamma(P) equals zero." Is he right?

  • It's a style issue not a grammar issue. If most other papers in your field, or in that journal use this format, then it might be warranted. Otherwise, from a normal grammar or semantics point of view, it's rubbish. The reviewer might feel that it's clearer or easier to parse. I'm not sure that's true, but it might be. Commented Sep 11, 2015 at 11:36
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    If you're happy and you know it, then clap your hands.
    – RegDwigнt
    Commented Sep 11, 2015 at 12:00

1 Answer 1


"If X, then Y" is technically correct and sometimes less ambiguous than "if X, Y", but the latter is also acceptable English.

Consider a sentence with more than one condition before the consequence: "if X, Y, and Z, then W" is obviously clearer than "if X, Y, and Z, W". For this reason, writers in fields such as maths and theoretical computer science, which require great precision in logic while making very convoluted statements, often expect "if X, then Y" rather than "if X, Y". A sentence beginning "if X, Y" could go on to say "if X, Y, and Z, then W"; it's usually better to make it absolutely clear when you pass from the condition to the consequence.

As an amusing side note, you can often recognise mathematicians by their writing style even in informal writing, because they almost use use "then"s after their "if"s!

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