Scilling, your comments are always so immensely helpful. I can't thank you enough.
To be sure I understand, the imperative mood is basically when you're commanding someone to do something and the second person singular refers to "tú" as opposed to "sibh". Thus the headword "déan" is the same as in the expression "déan deifir!" (make haste!/hurry up!). Is that correct? And is there any particular reason that version becomes the sort of emblem for the other conjugated forms? Is it because the root is always visible or something along those lines? Thanks again in advance.
Yes, that’s correct. I suspect that that conjugation is used as the headword in modern dictionaries precisely because it’s essentially a root form. (Older dictionaries used the first-person singular indicative mood; for example, Dinneen’s 1904 dictionary used do-ghním as a headword instead of déan ; déanaim used to be only a dependent form.)
Not necessarily all new words are highlighted as new words in this course. Especially with plurals, genitives and verb forms you will see surprising versions coming.
It is one of the real minor shortcomings of this course due to it being implemented by volunteers. In my personal opinion this in no way reduces the great value of this course.
Don't worry: After the third miss you will usually remember, and actually protesting / reacting will make it easier to remember.