Why do we say a Christmas tree and not an Christmas tree?

Answer 1

Because it depends whether it is a vowel or a consonant. For example 'a apple' is wrong. 'an apple' is correct. 'a book' is correct. 'an book' is wrong. The word that begins with a vowel like orange, apple, elephant, then we will say an before it but with a consonant like book, computer, table we say a before it. Just like that xmas tree begins with a consonant, therefore we say 'a' before it, not 'an.'

Actually, it is whether it is a vowel *sound* rather than an actual vowel, but otherwise, dead on. For instance, you would say "an hour" but "a hippo" ... it depends on whether the words starts would a vowel sound.