elven vowels

how can you tell, when writing tengwar, when to use the Y that looks sort of like a bass clef in music w/o the dots or a lopsided backwards C, and when to use the one that looks like an A without the horizontal bar?

[Nilpaurion Felagund]

The bass clef-like sign becomes vocalic "y" in the mode of Beleriand(the Moria-gate inscription) The "a" w/o the horizontal bar is a in the same mode. That's right, is it? I need someone more expert than I am in these things.

[Gwaihir the Windlord]

I cannot write in Tengwar. I believe there is a section in the Appendices to Lord of the Rings, however, with a fairly explanatory chart that should help you with it?

When to use... remember that it isn't a strictly-ruled form of writing, unlike our own, as it describes in Appendix B. It just has to sound like the word you're trying to write (although it's probably a good idea to be consistent whenever you write in Tengwar), basically; that might be of assistance to you if you're still in any way puzzled as to how to use a system of writing with no set spellings...

I've mostly been using the backwards C for the vowel Y, I, and long E, and the A w/o horizontal bar for everything else. I once took an etymology class and the teacher said that vowels get changed often, but the words still sound similar, so I guess it doesn't matter that much. But a's don't sound very much like o's, or u's, so sometimes I will write something and not be able to read it afterwards.

I looked in the Appendixes but they weren't very much help.

-Dilly