Learn Finnish: Olen kassalla jonottamassa. - I am queuing at the checkout.

Both can translate to "I am queuing", but using olla (to be) + the third infinitive inessive (-massa/-mässä) emphasizes that you are right in the middle of the action at this exact moment. It's the closest Finnish equivalent to the English continuous tense ("I am currently doing").

How is the word jonottamassa formed from the base verb jonottaa (to queue)?

To form the third infinitive, you take the strong stem of the verb and add -ma (or -mä), followed by a location case ending. Here, we take the strong stem jonotta-, add -ma, and then the inessive case ending -ssa ("in"). So: jonotta- + -ma- + -ssa.

Why is it kassalla and not kassassa?

The adessive case (-lla/-llä) translates to "at" or "on". Finnish uses it for points of service or open areas, like a checkout (kassa), a kiosk, or a bus stop. If you used the inessive case (kassassa), it would mean you are physically sitting inside the cash register!

Is the word order strict? Could I say Olen jonottamassa kassalla instead?

You can absolutely say Olen jonottamassa kassalla. Finnish word order is flexible. Placing kassalla before the main action word (jonottamassa) is just a very common and natural way to flow the sentence, slightly emphasizing where you are before stating exactly what you are doing there.