Learn Danish: Hvorfor købte hun ikke nederdelen? - Why did she not buy the skirt?

In Danish questions, the verb comes right after the question word. This pushes the subject (hun) to the third position. In main clauses, adverbs like ikke always come immediately after the subject, giving us the strict order: question word + verb + subject + adverb.

How would I change this sentence to say 'Why did she not buy the one that was red?' using this lesson's grammar?

You would replace nederdelen with the determinative construction den, der or den, som (since nederdel is a common gender 'en' word). The sentence becomes Hvorfor købte hun ikke den, der var rød?

What is the root word of nederdelen?

The root word is en nederdel (a skirt). It is famously a very literal Danish word, made up of neder (lower) and del (part). The -en at the end is the definite suffix, making it 'the skirt'.

Why doesn't Danish use a word for 'did' in this past tense question?

English uses the helper verb 'did' to form past tense questions ('Why did she not buy...?'). Danish doesn't do this. Instead, it simply uses the past tense of the main verb itself, which is købte (bought). Literally, the sentence is structured as 'Why bought she not the skirt?'.