Learn Finnish: Joskus palvelu on hidasta. - Sometimes the service is slow.

The lesson is about -sti adverbs like hitaasti (slowly), which describe how an action is done. However, in this sentence, we are describing a noun (palvelu, service) rather than an action. Therefore, we need the adjective hidasta (slow).

Why is hidasta in the partitive case here instead of the basic form hidas?

In Finnish, when you use the verb olla (to be) to describe an uncountable noun or abstract concept like palvelu (service), the describing adjective must be in the partitive case. If the subject was a specific, countable thing like a car, you would use the basic form: Auto on hidas (The car is slow).

If I wanted to change the sentence to say 'The service works slowly,' how would I form the adverb?

You would use the adverb suffix -sti. To do this, find the strong stem of the adjective hidas, which is hitaa-, and add -sti. The sentence would then be: Palvelu toimii hitaasti.

Is the word joskus related to the -sti adverbs we are learning?

No, joskus (sometimes) is a built-in time word. While many adverbs of manner are created by adding -sti to an adjective (like nopea becoming nopeasti), common time words like joskus, usein (often), and aina (always) are their own distinct words and don't use this suffix.