Learn Finnish: Tarjoilija oli tänään todella töykeä asiakkaalle. - The waiter was really rude to the customer today.

Adjectives describing interpersonal behavior toward someone, such as töykeä (rude), ystävällinen (friendly), or vihainen (angry), require the target of that behavior to be in the allative case. In Finnish, you are polite or rude 'onto' someone.

What is the stem of asiakas, and why are there two K's in asiakkaalle?

The noun asiakas belongs to the word type ending in -as. To find its stem, you replace the -s with an extra vowel, creating the basic stem asiakaa-. Because this process changes the syllable structure, it triggers consonant gradation from the weak single k to the strong double kk, resulting in the final stem asiakkaa- before the -lle ending is added.

Could I use tosi instead of todella here?

Yes, absolutely. Tosi töykeä means exactly the same thing. Todella is the standard form that works perfectly in both spoken and written Finnish, while tosi is extremely common in everyday speech but generally avoided in formal writing.

Why is asiakkaalle at the end of the sentence rather than next to the verb?