Learn Dutch: Ik heb een grote boekenkast gemaakt.

Boekenkast is a compound noun made of boek (book) and kast (case or cupboard). The rule for the linking -en- is that if the first noun only has a plural ending in -en (like boeken), we use -en- to glue the words together.

Why do we say een grote boekenkast and not een groot boekenkast?

The gender of a compound noun always comes from its last part. Because kast is a de-word (de kast), boekenkast is also a de-word. Adjectives before a de-word always get an -e added to them, even when they come right after the word een.

When adding the -e to groot, why does it lose an o to become grote?

This is because of the Dutch spelling rules for open syllables. When you add the -e, the word splits into syllables as gro-te. Since the first syllable ends in a vowel, the long o sound is naturally preserved with just one o. Writing it with two would break the spelling rules.

Why does the verb gemaakt go at the very end of this sentence?