Learn Dutch: We kopen vijf bomen. - We are buying five trees.

This is because of an important Dutch spelling rule. When you add -en to make a plural, the word breaks into syllables: bo-men. Because the first syllable ends in a vowel, it is an "open" syllable, meaning the 'o' automatically keeps its long sound. Dutch drops the second 'o' because it isn't needed anymore to show that the vowel is long.

Why do both the verb (kopen) and the noun (bomen) end in -en?

In Dutch, -en is a very busy ending! For nouns, adding -en is the most common way to make them plural (like adding '-s' in English). For verbs, plural subjects like we take the infinitive (dictionary) form of the verb, which also almost always ends in -en.

I know the "I" form of to buy is ik koop. Does kopen lose an 'o' for the same reason as bomen?

Exactly! The root of the verb has a long 'o' sound (koop). When you add the -en ending for the "we" form, the word splits into syllables: ko-pen. The 'o' is now at the end of an open syllable, so it naturally sounds long. Because of this, the extra 'o' is dropped to follow the exact same spelling rule.

The translation says "are buying", but there is no word for "are" in the Dutch sentence. Why?