Form, usage, convention, convenance are comparable when they mean a fixed or accepted way of doing or sometimes of expressing something.
Form can apply to a prescribed or approved way of behaving, method of procedure, or technique in any sphere of activity where correctness or uniformity of method or manner is thought essential.
Usage implies the sanction of precedent or tradition and often designates a form preserved out of respect for a class, profession, or religion.
Convention often replaces form especially in application to social behavior, where it stresses general agreement and therefore applies to some set way of doing or saying something that is sanctioned or believed to be sanctioned by general unquestioning acceptance.
Convenance is a somewhat literary word applied to social conventions especially regarded as essential to propriety or decorum.