Learn Finnish: Miksi asiakas ei puhu kohteliaasti? - Why does the customer not speak politely?

The base adjective is kohtelias (polite). To add the adverb suffix -sti, you must attach it to the word's vowel stem. For words ending in -as, the stem ends in -aa-. So, kohtelias becomes the stem kohteliaa-, and then we add -sti to get kohteliaasti.

Could we just say ei puhu kohtelias instead?

No, because kohtelias is an adjective meaning polite, used to describe nouns (like a polite customer). Since we are describing how the customer speaks (a verb), we must use the adverb form kohteliaasti (politely), just like adding -ly in English.

Why is the verb puhu short, instead of the normal puhuu?

This is a negative sentence. When using the negative word ei, the main verb that follows must be in its negative form. For the verb puhua (to speak), you take the positive he/she form puhuu and drop the last vowel to get puhu.

Is there a trick to remembering the word asiakas (customer)?

Yes! It comes from the word asia, which means a matter, thing, or business. The ending -kas is a suffix that often means having something. So an asiakas is basically someone who has business with you: a customer.