Questions & Answers about Testa rubra est.
Because rubra has to agree with testa in gender, number, and case.
- testa is feminine
- testa is singular
- testa is nominative
So the adjective must also be feminine singular nominative, which is rubra.
The other forms would be used with different kinds of nouns:
- ruber = masculine singular nominative
- rubra = feminine singular nominative
- rubrum = neuter singular nominative
Testa is nominative singular.
It is nominative because it is the subject of the sentence: the thing being described.
It is singular because it refers to one thing.
A learner can often spot this because testa is a first-declension noun, and the nominative singular of many first-declension nouns ends in -a.