Questions & Answers about Þetta var hugmyndin þeirra.
Why does the word þeirra come after the noun instead of before it like in English?
In Icelandic, possessive words like þeirra (their) usually go right after the noun they belong to. You are structuring the sentence to say: the idea of theirs.
Why does hugmyndin have the definite article -in attached to it if we are saying their idea?
This is a very common pattern in Icelandic. When you use a possessive pronoun like hans, hennar, or þeirra, the noun almost always takes the definite article. So hugmyndin þeirra literally means the idea their.
Does þeirra change its ending because hugmyndin is a feminine noun?
No, it does not. The word þeirra means their. Unlike adjectives, the possessive words hans (his), hennar (her), and þeirra (their) are invariable. They never change their shape to match the noun, no matter the gender or number of the thing being owned.
Why do we use the neuter word þetta (this) when hugmynd is a feminine noun?
When pointing something out or introducing it with phrases like this is or this was, you always use the fixed neuter form þetta. The gender of the noun that comes later in the sentence does not change this introductory word.