Learn Finnish: Asiakas halusi viipyä kaksi yötä, sillä majoitus oli viihtyisä. - The customer wanted to stay for two nights, for the accommodation was cozy.

To express how long an action lasts, Finnish uses the object cases. With numbers greater than one, the number itself takes its basic nominative form (kaksi) and the noun following it is in the partitive singular (yötä). Notice that no extra word for "for" is needed when expressing duration.

What is the difference between using sillä and koska for "because"?

Both words give a reason, but they behave differently in a sentence. Koska is a subordinating conjunction and can be used at the very beginning of a sentence. Sillä is a coordinating conjunction, functioning much like the English word "for," and can only be used in the middle of a sentence to connect two clauses.

Could I use jäädä instead of viipyä to mean "to stay" here?

They have slightly different focuses. Jäädä means to remain somewhere instead of leaving, and it requires a location in the illative case (like jäädä hotelliin). Viipyä means to linger or spend a certain amount of time, which pairs much better with a duration phrase like kaksi yötä.

How is the noun majoitus (accommodation) formed?