Learn Finnish: Ystävä, joka maksoi laskun, on rehellinen ja luotettava. - The friend who paid the bill is honest and reliable.

Joka refers back to ystävä (friend). Inside the relative clause joka maksoi laskun, the word 'who' is the subject doing the paying. Because it is the subject of its own clause, it takes the basic nominative form.

Are the commas around joka maksoi laskun mandatory?

Yes! In Finnish, you must always separate a relative clause from the main sentence with commas. While English sometimes drops them, Finnish always requires a comma before words like joka.

Why is the word for bill written as laskun instead of the basic form lasku?

Laskun is the object of the verb maksoi (paid). Because the friend paid the entire bill, the object gets the -n ending to show it was a complete, finished action.

The word luotettava looks like it comes from a verb. Is that right?

Spot on! Luotettava (reliable) comes from the verb luottaa (to trust). Adding the -tava or -tävä ending to a verb creates an adjective that means something can be done, so luotettava essentially means 'trustable' or 'able to be trusted.'