"The deer has a tail."
Translation:Het hert heeft een staart.
From my 16 years experience with Dutch, I dare to say that the given translation is wrong. It should be' De hert heeft een staart'. There is no logical rule for when you should use 'de' or 'het'. After some time you should be able to guess which one you should use. And by the way, there are many Dutch people who tend to say it wrong themself.