Another approach:
The indefinite article "ein
" has to be declined in congruency with the noun "Zeit
". To decline "ein
" correctly, you need to determine:
- the gender
- the case
of the noun. The grammatical gender of "Zeit
" is female. And this fact rules out "einen
" as option; see this overview regarding how to decline "ein
". So, "einen
" can't be correct in connection with "Zeit
", independent of the case.
So, you finally have to select between "eine
" and "einer
". Here, you'll have to look at the case of the noun (see splattne's answer), so "einer
" is correct.
To pick up your comment regarding "lang
":
"lang
" is quite difficult to handle: just compare the declination table for "ein
" cited above to that overview of the different forms of "lang
".
In this case, you need the first three tables for the "positive" (neither comparative, nor superlative) form of "lang
". Here, we need the third table, because there's "ein
", so it's a case of mixed inflection. The noun is singular female dative, so the correct version is "einer langen
".
ein
andlang
declensions. – Stephane Rolland Sep 01 '12 at 20:20