The reader does in fact not say buys ten cookies, but bought/has bought ten cookies.
- emō: I buy; emit: s/he buys
- ēmī: I bought; ēmit: s/he bought
Vowel length matters! Compare pax to pāx, English /hit/ to /hi:t/ (sorry about the slightly wrong IPA), German isst to ist or Pollen to Polen, Norwegian hatt to hat.
As I have stated before: Macrons should be included the way accents are in Greek. Technically this could be solved the same way as in German, where they have included clickable letters for ä, ö, ü and ß and as in Greek where you get told to mind the accents if you left them out. Not including them is poor pedagogyand this should be adressed by the developers. We do, after all, pay for this service (a lot of us, that is).
First declension nouns end in -a in nominative and vocative singular, such as agricola, -ae, fēmina, -ae or puella, -ae. Neuter nouns are special: nominative, vocative and accusative are always the same, and in plural, irregardless of declension, all three cases always end in -a. I hope this helps.