Why is it "Was fehlt Ihnen?" and not "Was fehlen Ihnen?"

Question

When using the formal form of you, "Sie", we conjugate the verb in the third person plural, regardless if the "Sie" refers to a singular or plural subject. "Ihnen" is the dative form of "Sie".

So why do we use the third person singular in this case even though we are still using "Sie", just in its dative form? When using another verb: "Was vermissen Sie?" not only do we not use the dative case but we use the third person plural as expected. Can someone explain the difference between these two sentences?

Answer 1

Don't let word order fool you. In German, word order is the least reliable hint on the function of an item in the clause.

Was fehlt Ihnen?

In this sentence, was is the subject and Ihnen is the dative object. The verb conjugation follows the number and person of the subject. Was is third person singular.

Was vermissen Sie?

In this sentence however, was is the accusative object and Sie is the subject. Again, the verb conjugation follows the number and person of the subject. Sie is third person plural.

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This difference is because of the verbs. The verb fehlen is intransitive. It's even one of those verbs that cannot have any accusative object. Unlike schlafen for example which accepts various types of sleep as its accusative object. But fehlen has none. Never. If there is ever an accusative in a clause with fehlen, its an adverbial accusative that tells how long the subject is missing.

Answer 2

The number of the verb (singular or plural) depends on the number of the subject.

So in order to tell whether fehlt (singular) or fehlen (plural) must be used one needs to know the subject.

As you already noticed, Ihnen is the dative form of Sie. Because of this, Ihnen cannot be the subject. The subject must be in nominative case.

Having ruled out Ihnen, the interrogative pronoun was is the only candidate for the subject. Was is singular. This means that the verb must be in singular. Which is why it has to be fehlt, not fehlen.

Answer 3

As noted by @Janka and @RHa, was is the subject. The question of the number of was is not trivial. For example, the answer could be

Uns fehlen noch Gläser.

The requested meaning of was can therefore also be plural. Still, was "works" in the sentence singularly. The singular here is to be understood as number-neutral, just as jeder is understood as gender-neutral in normal German [okay, not for the gender sensitive people]. To express that one thinks of was more in terms of a plural meaning, one could ask, for example:

Was fehlt Ihnen hier alles? / Was fehlt hier noch alles?

That would be comparable to

Wer kommt alles zur Hochzeit?

to express that not only one person is thought of here. Nevertheless, wer works singularly – by the way, women are also meant, although the pronoun wer looks masculine. In short: Singular indefinite and interrogative pronouns (jeder, keiner, mancher, …, wer) tend to be taken number-neutrally (and gender-neutrally), with the masculine being the gender-neuter form. Some of these pronouns work syntactically in the plural (alle, mehrere, etliche …), but was and wer work (syntactically) in the singular.