"Welcher kluge Mann!" vs. "Welch kluger Mann!"

Question

My German teacher says that both of the above phrases are correct. However, based on what we have learned thus far, since "welch" is a 'der word', it takes a "-er" ending in the nominative case. Since "klug" is an adjective following a 'der word', it must take a "-e" ending in this case.

There are a few things I don't understand in this sentence.

1. Are both versions correct? Why or why not?

2. If so, is this a special case, or is there a general rule for when this is permissible? (The endings act as if "welch" is an 'ein word' in the second sentence.)

3. This one's not as important, but since "welch" translates to "which", why is this sentence an exclamation(What a smart man!), rather than a question (Which smart man?). How would I ask "Which smart man?"

Thanks in advance for your help!

Answer 1

They are both (technically) correct, but they mean different things. (Actually, the exclamation mark is not: it would have to be a question mark in the first example.)

Welcher kluge Mann?

means "Which smart man?" (There are several).

Welch kluger Mann!

means "What a smart man!" (I admire him greatly.)

Answer 2

The uninflected form welch is used in the expression welch ein, and sometimes before attributive adjectives.

Sie wunderte sich, welch ein dummer Mensch dies getan haben könnte.
Sie wunderte sich, welch dummer Mensch dies getan haben könnte.
Welch herrliche Aussicht!
Welch eine herrliche Aussicht!

[...]

Welche unerwartete Überraschung!
Welch unerwartete Überraschung!

Whenever you use the uninflected form, you need to take the strong inflection of the adjective.

Welcher as interrogative pronoun is always inflected and you must apply the weak inflection to the adjective that follows welcher.

You find a complete explanation and the quote above on canoo.net.