If I’m not mistaken, the formal imperative reads like this:

<Infinitive Verb> + Sie + … ( + bitte).

In case of the verb sein the infinitive is just sein itself. So on what grounds this sentence is correct?

Seien Sie vorsichtig.

I did not figure it out after a little lookup.


This does indeed look like an exception. I am not aware of any other verbs where the imperative polite form does not match the infinitive (but I may just not have thought of it). In any case, the polite (Sie-form) imperative of sein is indeed seien.

Canoo.net claims that there are only two imperatives: singular and plural. For sein, the singular imperative would be sei and the plural one seid. They continue to say that to make a polite request, one must resort to conjunctive I of the third person plural (capitalised to be Sie). Since they state no exceptions, it seems that ‘seien Sie’ can be perfectly rationalised with this rule. Again, I am not aware of any other verb whose conjunctive I form in third person plural does not match its infinitive.

Canoo.net’s definition does suffer from the fact that an imperative statement comes with a different word order, but it seems consistend and rational.

  • 2
    Ahh, once more sein proves to be the exception to every rule. Most irregular word there is, in every language that has it.
    – Chieron
    Sep 2 '16 at 15:06

The infinitive "sein" and the 3rd plural present subjunctive "sie seien" are etymologically identical: both continue MHG sin. The difference between "sein" and "seien" is purely orthographic.Moreover "seien" is normally pronounced just like "sein" ("sei'n Sie so gut..."); the bisyllabic pronunciation of "seien" is spelling-based.

  • Your etymological argument sounds plausible (I am just a layman). I heard bisyllabic pronunciation of seien too often to attribute it to people pronouncing the spelling, though.
    – Jan
    Sep 4 '16 at 20:30

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