"rug" is de: de rug (back). This one you just learn — no rule points to de here. The plural is de ruggen.

Is it de or het rug?

de rug

de-word

back

/də rʏx/

Plural

de ruggen

Why de

This one you just learn — no rule points to de here.

In a sentence

Ik heb pijn in mijn rug.

I have pain in my back.

More de-words

The de/het checker covers one word. The full platform covers vocabulary, grammar and all five DUO sections.