"I'm Spanish" or "I'm a Spanish"?

Solution 1:

Spanish is an adjective, so no article. A Spanish man is a Spaniard. Note that for many other nationalities, the form of the adjective and the noun is the same:

• American, an American
• German, a German
• Italian, an Italian
• Russian, a Russian
• Chinese, a Chinese
• Japanese, a Japanese
• Greek, a Greek

I have a feeling that for most nationalities the adjective and the noun have the same form, and only in a few cases are the forms of the adjective and noun distinct, e.g.:

• Danish, a Dane
• English, an Englishman
• French, a Frenchman
• Irish, an Irishman
• Scottish, a Scot(sman)
• Spanish, a Spaniard
• Welsh, a Welshman

Solution 2:

Yes, adding an "a" before Spanish, would be wrong. That could be reworded as I'm a Spaniard. See http://dictionary.cambridge.org/dictionary/british/spaniard?q=Spaniard

Solution 3:

"Spanish" is an adjective, not a noun. You cannot use either an indefinite article with it. You can say:

• I am Spanish
• I am a Spaniard

However, the two sentences are not equivalent. The first implies descent, while the second implies citizenship or nationality. I could very easily be of English descent, and yet be a Spaniard, if I changed my citizenship, for example.