Learn Finnish: Jos lopetan työt aikaisin, minulla on aikaa varata hieronta. - If I finish work early, I have time to book a massage.

This is the standard structure for expressing that you have the resource to do something. When the resource is abstract and uncountable, like time, it takes the partitive case: minulla on aikaa (I have time). It connects directly to the first infinitive, varata.

Why is the object hieronta in the nominative case instead of the genitive (hieronnan)?

Because the main clause is a possessive sentence (minulla on). Possessive sentences do not have a grammatical subject in the nominative case. When a clause lacks a nominative subject, its total object takes the nominative case instead of the genitive. This rule applies even when the object is attached to an infinitive verb like varata.

Why is work expressed in the plural as työt? Could I say jos lopetan työn?

Using the singular työn means finishing one specific task or project. However, when talking about your daily job, workday, or shift as a whole, Finnish naturally uses the plural työt. Saying lopetan työt is the idiomatic way to say I finish work.

What is the difference between aikaisin and ajoissa?

Aikaisin means early, as in earlier in the day or earlier than usual. Ajoissa means on time or with time to spare before a specific deadline. Since the sentence talks about finishing the workday earlier than normal, aikaisin is the correct choice here.