"The horses eat."
Translation:De paarden eten.
In the tips it says:
"Always maintain vowel length. If a vowel sounds long, make sure it keeps sounding long (for instance, schaap becomes schapen). If a vowel sounds short, keep it short for instance, kat becomes katten)."
I think you're mixing up the above with a spelling rule. The pronounciation of the a/aa in schaap and schapen is the same. In both cases this is the long a. The short a is used in words like dat, vlag and Galatasaray.
The spelling rule is explained under "2. Open and closed syllables" here: https://www.duolingo.com/comment/3733043