Learn Finnish: Me siivosimme ison tehtaan koneilla. - We cleaned the big factory with machines.

The base word is kone (machine). Words ending in -e double the vowel in the stem, making it konee-. To form the plural adessive, we add the plural marker -i- and the adessive ending -lla. When the plural -i- is added to konee-, one e drops out, resulting in koneilla.

Doesn't the adessive case usually mean "on" or "at"? Why use it for "with machines"?

You are right that the adessive ending (-lla/-llä) often translates to "on" or "at". However, Finnish also uses it to show what tool, instrument, or method was used to do an action. Here, koneilla tells us by what means the cleaning happened: "with machines".

Why do ison and tehtaan end in -n?

They make up the total object of the sentence. Because the cleaning of the factory is a completed, whole action, the object takes the accusative case, which looks exactly like the genitive case (ending in -n) for singular words. If the cleaning was unfinished or ongoing, you would use the partitive case instead (isoa tehdasta).

What is the base word for tehtaan and why does it look so different?