[englishstudent]

Thank you Casiopea for your response.

>"It was" contracts to 'twas, which is no longer in use in Modern English. "It was" wouldn't >contract to "It's" because it has a word-initial consonant w-. There are only two possibilities >for "It's": It is, It has. The contraction is possible because "is" has a word-initial vowel, i-, >and "has" has word-initial h-.

I'm sorry I could not catch the difference between:
"has" has word-initial h-.
and
("was" ) has a word-initial consonant w

Thanks