Learn Finnish: Pelissä on pysäyttämätön kone. - In the game there is an unstoppable machine.

It comes from the verb pysäyttää (to stop something). To form the negative participle, you take the strong stem of the verb, which is pysäyttä-, and add the suffix -mätön. This creates an adjective meaning "unstoppable" or "unstopped".

Why is the suffix -mätön here instead of -maton?

This is due to vowel harmony. Because the root verb pysäyttää contains the front vowels y and ä, the suffix must also use front vowels. Therefore, we use -mätön.

Could I use the other verb for stopping, pysähtyä, to make this word?

No, because pysähtyä means to come to a stop on your own. To say something is "unstoppable" by an outside force, you must use the transitive verb pysäyttää (to stop something else), which gives us pysäyttämätön.

Why are the words pysäyttämätön kone at the very end of the sentence?

This sentence uses the Finnish existential structure, which is how you express "there is" or "there are" in English. These sentences typically begin with the location (pelissä), followed by the verb (on), and place the new subject (pysäyttämätön kone) at the end.