What is a word made up of 4 letters yet is also made up of 3

Question: What is a word made up of 4 letters, yet is also made up of 3. Sometimes is written with 9 letters, and then with 4. Rarely consists of 6, and never is written with 5.

Question: What do you call a sad bird?

Question: What can be broken but never forgotten?

Question: Where do all the letters sleep?

Question: What tastes better than it smells?

Question: People see through me, but none pass through without killing me. I can be anywhere, and can be seen double or single. What am I?