Learn Finnish: Tämä tarjous on kai hyvä. - This offer is presumably good.

Both can translate to "maybe" or "probably," but they have a slightly different feel. Ehkä is a neutral "perhaps" or "maybe." Kai is more like "I suppose," "presumably," or "probably," showing that the speaker assumes it's true but isn't completely sure.

Where should I put kai in a sentence?

Little adverbs and particles like kai almost always jump to the spot right after the main verb. Here, it sits directly after on. You wouldn't normally start a sentence with kai.

The word tarjous ends in -us. How does it change if I want to add an ending, like saying "in this offer"?

Nouns ending in -us or -ys belong to a specific word type. Their stem changes to -ukse- or -ykse- before you add most grammatical endings. So, "in this offer" becomes tässä tarjouksessa.

Why isn't the adjective hyvä in the partitive case (hyvää) here?

In a basic "A is B" sentence, the describing adjective stays in the basic nominative form if the subject is a single, countable thing. An offer (tarjous) is treated as a countable noun here. If the subject were an uncountable substance like coffee, you would use the partitive: Tämä kahvi on hyvää (This coffee is good).