NaftaliFri1 is right, of course! If anyone would ever like a longer semi-technical/jargony explanation to a similar question, you can check out the following comment thread. Otherwise, it's best to just go with what NaftaliFri1 said and continue focusing on learning Hebrew as Hebrew! :-) https://www.duolingo.com/comment/17445579
Yes. But this is only assuming a well though-out sentence (e.g. in writing). In speaking, you can perfectly well use a "sub-optimal" order and make up for it in the phonetic stress. If you want to stress who the teacher is, it's more elegant to say אני לומד עברית מאבא שלי, but you might instead say אני לומד מאבא שלי עברית and phonetically stress אבא; it will sound perfectly natural.
Earlier stages of Hebrew had geminated consonants (like Finnish or Italian) where they were spoken with a longer duration. For example if you take מֶ֫לֶךְ ['melekh], if you put the article before it, you got הַמֶּ֫לֶךְ [hammelekh], in syllables ham-me-lekh. The proposition מ did the same thing: מִמֶּ֫לֶךְ "from a king", mim-me-lekh. But some consonants could not be lenghtened, so the vowel changed to a long one: מֵאִישׁ "from a man" me-ish. Modern Hebrew has lost consonantal gemmination, but traditional voweling still follow the rules, which are caused by the older phonetic stages of the language.
Well, the names of close relatives usually omit the definite article. The Hebrew Academy even writes: הַצֵּרוּף הַמְּיֻדָּע ״הָאַ֫בָּא״ אֵינוֹ תִּקְנִי the combination with the definite article HA'ABBA is not correct/standard. That the Academy writes this means of course that some folk say this ;-)