"The new house resembled the old one."
Translation:Det nya huset liknade det gamla.
The definitive form means there's a reference to a specific thing, not any one of a thing. For example, "the house" is definitive while "a house" is not.
In English, adjectives and nouns don't change when they are definitive, but in Swedish they do. So it's "the new house" and "a new house" in English, but "det nya huset" and "ett nytt hus" in Swedish.
In Swedish, when you have an article modifying a definite noun, you need both the separated article (den for en-words, det for ett-words, or de for plural) AND the definite suffix. That's why det nya hus is incorrect. It might seem rather redundant at first, but I promise that you'll get the hang of it quickly.