A morpheme is a minimal meaningful unit. To use your examples,
m- <a number prefix>
A noun can be either singular or plural (e.g., English has book, books.That is,-s is added to form plural nouns).
In the language you're working with, m- is added to Class 1 nouns to make them singular.a- <an agreement marker>
The subject must agree in number with its verbs (e.g., English has The book is big; The books are big. The verb agrees in number with its subject.
In the language you're working with, if the subject is a Class 1 singular noun, then add a- to its verb.