I'll do my best to keep the answer short, and if I leave something out, please add on! :) The modal verbs in English are Can, Could, Would, Should, May, Might, Need to, Must, Have to, Have got to. Please note that of these verbs, need to, have to, and have got to conjugate while the others do not. These verbs go in front of the main verb in a sentence and usually add a tone of suggestion, ability, or a lack of ablity to the verb. For example: I swim. (As a hobby or for exercise, general statement about what you like to do) I can swim. (The emphasis here shows an ability to swim, although you might not actually do it that often.) I could swim (Here you are able to except that there is something preventing you from swimming. I could swim right now if I didn't have to be at work in an hour) I would swim (Similar to

I've found the word "modality" very vague as well. Basically, if you use a modal verb then you are talking about the action in theory. It doesn't actually happen. Very old grammar rules call modal verbs "potentials".