Modality refers to a set of related concepts primarily involving the attitude of the speaker of a sentence towards the reality of a particular assertion. What exactly that means is complicated and best illustrated with an example:
(12a) Tad programs computers for a living.
In the context of language-arts education, a certain prescriptivism is unavoidable. There is a written standard, like it or not, and there are social consequences to violating the standard in certain contexts.