Answer:
I am the word that has three syllables and twenty six letters.
What is the word that even in plain sight remains hidden?
I think you live beneath a roof that is upheld by me; I think you seldom walk abroad but my fair form you see; I close you in on every side you very dwelling pave and probably I'll go with you At last into the grave.
When I am caught I am thrown away. When I escape you itch all day.
I start in little but I end in full you'll find me in half and complete.
No matter how little or how much you use me, you change me every month.