Quote Originally Posted by grafrich View Post
"Еда на Дне рождения была очень вкусной"
"Еда на Дне рождения была очень вкусная."

Apparently both are correct. But the first one looks wrong to me; вкусной is the ending for genitive, dative, instrumental and prepositional. Isn't it supposed to agree in gender number AND case, with "Еда"?
It is Instrumental.
There are two ways of grammatical interpretation of that sentence:

"The food was tasteful." Here tasteful should agree with the subject and takes Nominative.

"The food was a tasteful food." or "The food was a tasteful one."
Еда была вкусной [едой].

Here tasteful should agree with the skipped but implied object and takes Instrumental.

In Russian when the object is the same word as the subject (where in English reference "one" is used) it is just skipped.