Learn Turkish: Parkta oturmak çok güzel. - Sitting in the park is very nice.

The suffix -mak turns the verb root otur (sit) into a noun meaning "to sit" or "sitting". Since the act of sitting is the subject of this sentence, it needs this verbal noun form.

The English translation says "Sitting", so why don't we use a continuous ending like -iyor?

In English, "-ing" is used both for actions happening right now (I am sitting) and as a noun (Sitting is nice). Turkish uses two different forms for these. To use an action as a noun subject like we do here, Turkish uses the infinitive -mak or -mek instead of the continuous tense.

Why is it parkta instead of parkda for "in the park"?

The locative suffix for "in" or "at" is usually -da or -de. But because the word park ends with the hard, voiceless consonant k, the d changes to a t to make it easier to say.

There is no verb for "is" in this sentence. Is it missing?

It is implied! In Turkish, when you want to say a subject "is" an adjective (like "is very nice"), you usually don't need a separate word for "is" in the third person. Just putting the adjective güzel at the end of the sentence does the job.