Confusing Latin Adjectives

I received this question:

Hello Mr. Thomas,

I have a very basic question. I think I’ve asked you this before but I can’t find the email. Somehow I have a mental block about this:

When you have a noun which you must decline why doesn’t the adjective describing the noun have the same ending?

For example:

1. una ovis nigra

Why isn’t it: unis ovis nigris

2. Oves albae

Why isn’t it: oves albes

What rule do the adjectives follow? I thought they were supposed to “agree.”

Here is my reply:

The problem is, you are using 1st, and 2nd declension adjectives to describe 3rd declension nouns.  This is pretty common.

For example unus is a 1st and 2nd declension adjective.  I can only have 1st, or 2nd declension endings.  Unus can have any of these endings:

But, unus can never have third declension endings.  None of these guys.  They are never getting back together.  Like ever. 

But, what if you want to describe one sheep?  You describe the third declension word, ovis, with a 1st declension adjective.  First declension because ovis is feminine…. una ovis nigra.  Since oves is plural, and feminine oves albae.

They do agree in gender, number, and case… but, once we add in 3rd declension, the endings no longer match.  Make sense?