Would מהיר be the singular for מהירים?
I wrote "my fast horses" and was marked wrong. What's the difference?
Well, הַסּוּסִים הַמְּהִירִים שֶׁלִּי my fast horses has another word order and an article before the adjective.
Could this also be "My horses hurry" or "... are hurrying"?
No. That would be הַסּוּסִים שֶׁלִּי מְמַהֲרִים
Sounds like a crass idiom.
Should "I have fast horses" be acceptable?
That would be:
"יש לי סוסים מהירים."
I tried "My stallions are fast." and it was marked wrong. Why should that not be correct?
A stallion is an unfixed adult male horse - a stud. That's rather specific. But, steed should work.
Why is there an extra yud in the shoresh
Well, the adjective מָהִיר quick [mahir] has the vowel [i] in it.