In this context, what exactly does that mean? Because if it isn't I generally am willing to eat pasta (which seems to be Makarna yerim) or I am currently eating this specific pasta (which is that ongoing present tense thing) then I'm not sure when/how you would use this sentence.
There are a few ways I have heard.
"Ben makarnayı yedim" is the most literal translation (still correct though). "Makarnayı yedim" is, I think, the more informal version, or like saying you have done it, like "I have eaten it" (like someone might have asked you to and you're telling them you did it). And "Makarnayı ben yedim" sounds like you're owning up to having eaten it when you weren't supposed to.
I hope this is accurate and clear but if anyone has extra information or thinks I have made a mistake, you're welcome to add to my explanation :)
Neither or both: It follows a vowel harmony
The accusative case is actually formed by just adding a single vowel, according to the last vowel in the word that is turned into accusative:
a,ı -> ı (markana-yı - the pasta)
e,i -> i (defter-i - the notebook)
o,u -> u (doktor-u - the doctor)
ö,ü -> ü (kuaför-ü - the hairdresser)
In addition, the following rules apply:
if the word already ends in a vowel, a "y" is inserted before the accusative vowel
if the word ends in a "hard" consonant (ç,k,p,t), the hard consonant gets softened to (c,ğ,b,d). Example: kaşık -> kaşı-ğ-ı - the spoon