Learn Dutch: Soms kopen wij verse melk van de boerin. - Sometimes we buy fresh milk from the female farmer.

It is formed by adding the suffix -in to the base word boer (farmer). Adding -in is a very common way to create female professions and personal nouns in Dutch, just like you get vriendin (female friend) from vriend or koningin (queen) from koning.

Why is the word order kopen wij instead of wij kopen?

Because the sentence starts with the adverb soms (sometimes). Dutch main clauses follow the verb-second rule, meaning the conjugated verb must be the second element. Putting soms in the first position forces the subject (wij) to move to the spot immediately after the verb (kopen).

Why does the adjective vers get an -e ending to become verse melk, even without an article like de?

Melk is a de-word. Whenever an adjective comes before a de-word, it almost always takes an -e ending. This rule still applies even when the noun is uncountable and has no article in front of it, which is exactly what happens here with fresh milk.

Could I use we instead of wij in this sentence?