... I have no idea what a positive grade present tense third person singular is. I know the tense and what person it's in, but what makes a sentence a positive grade versus a negative grade (and is it singular because there's only one subject, or what)? I'm not coming at this from the perspective of someone who's familiar with the nuances of English grammar.
It's singular because the subject is a single entity rather than for example a group which would make it plural (we, they etc).
Positive grade is just the normal form where everything works just as expected. It's the negative grade that introduces some additional inflections. In Dothraki when a sentence is expressing the negative there is also a marking added to the verb. In English you don't see a difference in the verb between "I'm riding" and "I'm not riding" other than the added negator "not". In Dothraki you would also change the verb in the negative so
Anha dothrak would become
Anha vos dothrok. This is true for most verb conjugations in the negative but not all. For example in the present tense in the third person (he, she, it) when the verb stem ends in a consonant the conjugation would be
-i in both positive and negative grade.
Me adakhi "She is eating" and
Me vos adakhi "She is not eating"