Learn Finnish: Myyjä auttaa minua ystävällisesti. - The seller helps me in a friendly way.

For words ending in -nen, the basic rule is that the -nen changes to -se- before you add most endings. From the adjective ystävällinen (friendly), we get the stem ystävällise-. Then we just add the adverb suffix -sti to get ystävällisesti (in a friendly way).

Could I just say auttaa ystävällinen instead?

No, because ystävällinen is an adjective. Adjectives describe nouns (like a friendly seller). To describe an action—how the seller helps—you must use an adverb. The -sti ending acts a lot like the English "-ly", turning the adjective into the adverb ystävällisesti.

Why does this sentence use minua instead of minut for "me"?

The verb auttaa (to help) is a partitive verb. This means it always requires its object to be in the partitive case, which is a rule you just have to memorize for certain verbs. Because of this, the basic pronoun minä (I) becomes the partitive minua.

Is the word myyjä (seller) related to a verb?

Yes, it comes from the verb myydä (to sell). By adding the ending -jä (or -ja depending on vowel harmony) to a verb, you create a noun for the person doing the action. It works exactly like adding "-er" in English (sell → seller).