One of the possible answers was "Péter does not run up to Kati, but only walks." Which word would imply a "but" in the sentence?
"Csak" pretty much covers both "only" and "but". But we could also say "hanem csak" to truly cover "but only".
As this is a negative, why is it still "felfut" rather than "fut fel"?
This is a special case, when you have 2 verbs, and they form a contrast.
Nem felfut, csak felsétál. Nem felmegyek a hegyre, hanem lejövök róla. Nem bemegyek, hanem kijövök.
I was wondering that myself.
I was wondering that too. Also I used the present simple tense in my English translation and was marked wrong . Why was only the present continuous acceptable?
So csak is treated the same thing as hanem.
nem prefix-verb, (hanem/csak) prefix-verb
This is correcting "doesn't" to "doest" but there is no option for me to report that
To Kati or to Kati's place?
there is a word which should be inserted to make it grammatically correct. We should rather say: "Péter is not running up to Kati; he is just walking up"