Learn Finnish: Sataa, joten me emme purjehdi. - It is raining, so we are not sailing.

In Finnish, weather verbs like sataa (to rain) do not need a pronoun subject like 'it' in English. You just use the verb directly in its third-person singular form.

The lesson is about kun (when). How would this sentence change if we used kun instead of joten (so)?

You could rephrase it as Kun sataa, me emme purjehdi (When it rains, we are not sailing). While joten links a cause and a result, kun makes the first part a dependent time clause. Both express a very similar idea!

Why is the verb purjehdi different from its dictionary form purjehtia?

This is a negative sentence, so the main verb must be in its negative (connegative) form. To find this, take the 'I' form of the verb (purjehdin) and drop the final -n, leaving purjehdi.

Why does the t in purjehtia change to a d in purjehdi?

This is because of consonant gradation. The negative form is built from the 'I' form of the verb. Because adding the -n for 'I' closes the syllable, the strong t softens to a weak d. The negative form keeps this weak stem even after the -n is dropped.