In the previous section, I briefly introduced you to the modal auxiliaries when I argued that will does not constitute a separate tense marker. To understand the function of modal auxiliaries, you need to know two related terms: modality and mood.
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.