21. What are they?

"The Perpendicular," "Suspended Macron," etc.

22. Is the letter y ever marked by Diacritical Marks?

It is, sometimes.

23. What marks are used for y?

Macron and Breve.

24. Give examples where y is marked with the Macron.

Spȳ, slȳ, stȳ, etc.

25. Give example where y is marked with the Breve.

Hy̆mn.