Learn Danish: Han opfandt telefonen for hundrede år siden. - He invented the telephone a hundred years ago.

In Danish, "ago" is expressed by placing the amount of time between the words for and siden. Here, the time period hundrede år (a hundred years) is sandwiched in the middle, creating the overall meaning "a hundred years ago".

Is opfandt an irregular past tense verb?

Yes, it is the past tense of opfinde (to invent). It follows the exact same irregular pattern as the common verb finde (to find), which becomes fandt in the past tense.

Why doesn't the word år have a plural ending to match "years"?

The word år (year) is a neuter noun that keeps the exact same form in the plural. So, whether you are talking about et år (one year) or hundrede år (a hundred years), the word remains completely unchanged.

Why isn't there a word for "a" before hundrede år?

Unlike English, which uses "a hundred", Danish commonly drops the word for "a" or "one" (et) before exact hundreds and thousands. You can simply say hundrede år directly.