The Part-of-speech morphology option will create affixes for an entire part-of-speech. The left side of the = symbol must be the abbreviated part-of-speech (as it is in the dictionary) and the right needs a - symbol to create either a prefix or suffix, depending where it's placed:
nm = -o
nf = -a
v = to-
The above example adds "o" to the end of any masculine noun, "a" to the end of any feminine noun, and "to" to the beginning of any verb.
Use the bar symbol | to create a a random choice between affixes:
v = -ar|-er|-ir
Sound changes & Conditional rules
More complex sound change and conditional rules are described in the custom words page.