Learn Dutch: Hij weigert bloemen te kopen, dus ik pluk er drie mooie. - He refuses to buy flowers, so I pick three beautiful ones.

Here, er acts as a quantitative pronoun meaning "of them". In Dutch, when you state a quantity of something previously mentioned (like bloemen) without repeating the noun itself, you must include er. So ik pluk er drie literally means "I pick three of them".

Why is there an -e at the end of mooie?

In English, you say "beautiful ones" to avoid repeating the noun, but Dutch uses the adjective on its own. Because mooie refers back to the plural noun bloemen, it keeps the same -e ending it would have if the noun were actually written there (drie mooie bloemen).

Why does the sentence use te kopen instead of just kopen?

The verb weigeren (to refuse) requires the word te before the infinitive of the following verb. It works exactly like English: "he refuses to buy", rather than "he refuses buy".

Why isn't the word order inverted after dus? Shouldn't it be dus pluk ik?