"What a beautiful yard."
Translation:Was für ein schöner Garten.
"Was für is an interrogative pronoun and synonymous with welcher, welche, welches and welch Duden It translates to "what [kind of]" or "what [a]" in English." found here: http://german.stackexchange.com/questions/10138/ist-was-fur-etwas-nominativ-oder-akkusativ. If you continue to read you'll see that in the example used in the original question it's an exclamation, just like here, so is nominative.