Interrogative Determiners: welcher, was für ein

German has two ways to ask about a noun in a question, and they mean genuinely different things. welcher/welche/welches asks which one? — pick from a set you both already know about. was für ein asks what kind? — describe the thing for me. English keeps these apart too (which car vs what kind of car), so the meanings will feel familiar. The trap is grammatical, not conceptual: in was für ein, the word für does not behave like the preposition you learned.

welcher — choosing from a known set

welcher points into a defined group and asks you to select one member. The speaker assumes the set is already on the table: which of these books, which of those trains, which of the available colors. It declines exactly like a der-word (the same endings as dieser).

In the last example, mit forces the dative, so welcher takes the dative-feminine ending -er (welcher Straßenbahn). The ending follows whatever case the rest of the sentence demands — just like any der-word.

The key sense of welcher is selection from the known. If you ask Welches Auto nimmst du? you are implying there are specific, identifiable cars to choose between. You are not asking what the car is like; you are asking the person to pick.

was für ein — asking what something is like

was für ein asks for a description or category: what sort, what type, what kind. The answer is an adjective or a class, not a selection.

A natural answer to Was für ein Auto fährst du? is Einen Kombi ("an estate car") or Ein altes, rotes ("an old red one") — a description. A natural answer to Welches Auto nimmst du? is Das blaue da ("the blue one there") — a selection. That is the whole distinction in practice.

The surprise: für does NOT govern ein

Here is the one piece of grammar that catches every English speaker. In was für ein, the word für is not acting as the preposition für (which normally takes the accusative). It is part of a frozen idiom was für ("what kind of"), and it has no power over the case of ein at all.

Instead, ein declines according to the role the whole noun phrase plays in the sentence. If the noun phrase is the subject, ein is nominative; if it is a dative object after another word, ein is dative — completely independent of für.

Look closely at the third sentence. There are two prepositions in play: mit and für. The case of einem comes from mit (which takes the dative), not from für. The für inside was für is inert. If für were doing its normal job, ein would be accusative — but it is dative here, proving that für has been switched off inside this idiom.

This is why grammarians call was für a frozen or lexicalized expression: the original prepositional meaning of für has bleached away. The same thing happens to ein in the plural, where it simply disappears — Was für Bücher liest du? ("What kinds of books do you read?"), because the plural of ein is nothing.

Common Mistakes

English speakers often map which onto welcher even when they mean what kind. If you want a description, you need was für ein.

The reflex is to make ein accusative because of für. But für is frozen here; the case comes from mit (dative).

Because there is no preposition governing it and the phrase is the subject, ein stays nominative. (This sentence also works as an exclamation — see wie and was für exclamations.)

There is no plural ein, so it simply drops out: was für + bare plural noun.

Key Takeaways

• welcher/welche/welches = which? (select from a known, identifiable set); declines like a der-word according to the sentence's case.
• was für ein = what kind? (asks for a description or category); the answer is a type, not a choice.
• Inside was für ein, für is frozen and does not govern ein. The case of ein comes from the noun phrase's role in the sentence (subject = nominative, object of mit = dative, etc.).
• In the plural, ein disappears: was für Bücher, was für Schuhe.