Learn Turkish: Sınavdaki sorular imkânsız görünüyor, halbuki cevaplar çok kolay. - The questions on the exam seem impossible, however, the answers are very easy.

You could, but halbuki adds a specific nuance. While ama just means a neutral 'but', halbuki translates closer to 'whereas' or 'but in reality'. It highlights that the second part of the sentence is a surprising truth that contradicts the appearance set up in the first part.

The lesson also covers oysa. Can I swap halbuki for oysa in this sentence?

Yes, you can use oysa here with the exact same meaning. Both halbuki and oysa are contrastive conjunctions used to compare two situations, especially when the second clause reveals the actual truth contrary to the first clause.

Why is the word sınavdaki used instead of just saying sınav soruları?

Sınav soruları means 'exam questions' as a general category. By using sınav (exam) + -da (in) + -ki (the one which is), sınavdaki sorular specifically points out 'the questions that are in the exam' you are currently looking at. The -ki suffix is a handy way to turn a location into an adjective.

Why is the verb görünüyor in the present continuous tense if the English translation uses the simple present 'seem'?