So like gostar, precisar require "de" after it? I cannot say "Eu preciso um cavalo"?
Yes, you have to use DE. But, if you have precisar+verb you dont add the preposition. I need to go = eu preciso ir
when should we use "de" and "do' with precisar?
do = de+o, that is, you have the definite, masculine article
I need money = eu preciso de dinheirop (general)
I need the money I lent you = eu preciso do dinheiro que eu te emprestei (specific)
Is de um =dum a common contraction? It accepts it.
"Dum" is also right, but it is not so common.
do you need of one horse? i think you wrong many times in you translation