Learn Croatian: Prodaja ulaznica počinje ujutro. - The sale of tickets begins in the morning.

Prodaja (sale) is an action nominal, which is a noun derived from an action verb. In Croatian, the object of that action—the thing being sold—takes the genitive case. This acts just like the word 'of' in the English phrase 'sale of tickets' and is known as the objective genitive.

Doesn't ulaznica just mean 'ticket' (singular)? Why is it translated as plural here?

The word ulaznica here looks identical to the nominative singular form, but it is actually the genitive plural. For many feminine nouns ending in -a, the genitive plural ending is also -a (pronounced with a longer vowel, though this isn't written). Context tells us it means 'of tickets' rather than 'of a ticket'.

I often see verbs for starting or stopping use the reflexive pronoun se. Why is it missing here?

The verb počinjati (to begin) can be used intransitively without se when the subject itself is starting, as with the sale here. While you might occasionally hear počinje se with other phrasing, using just počinje is the standard and most natural way to say that an event or process is kicking off.

Is there a difference between writing ujutro as one word versus two words (u jutro)?