Original sentence: The bigger and thicker the horns, the older the tahr is.
Corrected sentence (clearer): The bigger and thicker a tahr's horns are, the older it is.
Why this is better (easy explanation):
- We add a tahr's so we know whose horns we mean.
- We use are with horns because horns is plural.
- A simpler way to say it is: If the horns are big and thick, the tahr is older.
All three versions mean the same thing, but the corrected one is a little clearer.