Why does this skill contain a sentence in past tense which is taught no less than 10 skills after this one? Moreover, "können" is an irregular verb that is introduced in this skill and I barely had a chance to learn its conjugation - "konnte" therefore makes it quite confusing.
When you have a conjugated verb and an infinitive at the end, the nicht tends to go before the infinitive verb at the end. Here is an article that might help with deciding where nicht goes
Not really. Couldn't suggests one has completed an effort unsuccessfully. Cannot, at least implies that for the time being you are not being successful, but does not concede the quest is finished yet. A lot of verbiage there; but in technical terms while "couldn't" and "can't" might be used by some interchangeably, they, in the purest sense of grammar are not equivalent verbs--there is a distinction from the one to the other.