"The horses and the sheep."
Translation:De paarden en het schaap.
There's a guide you can consult but basically you need to memorize which words are het words and which words are de words. This is a good memorization tool: http://www.memrise.com/course/356041/dutch-duolingo-de-or-het-vocabulary/
No, because double vowels and consonants (and open/closed syllables) are just an orthographic device to mark whether the vowel is long or short. The vowel itself doesn't change (except in a handful of exceptions, dag being the most notable one). Eg:
Naam (long /a:/) - plural namen; as it is an open syllable (na-men), the single vowel already denotes a long vowel.
Kat (short /a/) - plural katten; here the consonant is doubled to make it orthographically a closed syllable (kat-ten) so the vowel is read as short.
Paard (long /a:/) - plural paarden; as the root syllable is always closed because it ends in two consonants (thus paar-den), you need to keep the double vowel, otherwise it would suddenly denote a short vowel.