You don't need to use the partitive with "korjata". If the object is in partitive, that indicates that the action is ongoing (we are fixing x right now). If the object is in the genitive-looking accusative (sinä korjaat halvan tuolin) that means that the action is e.g. habitual (you fix x every Thursday). And yes, you can translate the verb "korjata" in more than one way. :)
The tips in the first lessons taught us how to conjugate a verb by removing the last vowel and then adding -n, -t, -VV, -mme, -tte and -vVt.
The verb "to clean" is korjata. If I remove the last vowel, it becomes korjat. In this case, "you are fixing" would be "sinä korjatt", which differs from the correct translation sinä korjaat.
Similarly, "to paint" is maalata. Just like before, by removing the last vowel it becomes "maalat". Then, "they are cleaning" would be "he maalatvat", but the correct form is he maalaavat.
Question: What rule do I have to follow when I conjugate a verb that ends in "ta"? Are there verbs that need another special consideration?