Interrogative adjectives

These are adjectives used in asking questions. There are basically two of them in German:

welcher (which) and was für (what sort of)

Welcher is declined exactly like dieser (i.e. like der,die,das) and takes its number, gender and case from the noun it is describing in the usual way.

Welches Buch ist das? Which book is that?

Welchen Bleistift hat er in der Hand? Which pencil has he got in his hand.

Mit welchem Zug fährt er nach Salzburg? By which train is he travelling to Salzburg?

Was für ein Hund ist das? What sort of dog is that?

Mit was für einem Kugelschreiber schreibt er? What sort of pen is he writing with?

Note that was für is uninflected - i.e. it does not change, but with singular words ein must be used in the appropriate case with the corresponding case ending, agreeing with the noun it describes.