While reading a English Syntax book, I wondered how I have to read a', a'', a''' in English. Would you tell me how to?
I'm guessing that you are referring to McCawley's numbering system for example sentences:
b. ... and so forth.
These are not actually English words but typographic symbols. Like the tree diagrams, they're not really intended to be spoken.
If you have to speak them, you may as J.R. suggests say a-prime, a-double-prime, etc. (which is a use borrowed from mathematics), or you may as FumbleFingers suggests say a-tick, etc. (but that is a British rather than US usage).
I'm most familiar with it in mathematics, where they are used as related designations of a variable a (or more commonly x, y, or z). I have always read them a-prime, a-double-prime. Haven't really come across a‴ before, but a-triple-prime makes sense. Wikipedia begins with this entry for Prime (symbol):
The prime symbol ( ′ ), double prime symbol ( ″ ), and triple prime symbol ( ‴ ), etc., are used to designate several different units and for various other purposes in mathematics, the sciences, linguistics and music.