Learn Icelandic: Húsið er á milli hárra trjáa. - The house is between tall trees.

The phrase á milli (between) is a preposition that always forces the words following it into the genitive case. That's why we don't use the basic dictionary forms here.

Trjáa looks very different from the basic word for "tree". Why is that?

The basic word for "tree" is tré. It is a neuter noun with a highly irregular plural. When forced into the genitive plural by á milli, the spelling changes significantly to trjáa.

Why does the adjective for "tall" have a double R in hárra?

The basic adjective is hár (tall). Because it needs to match the genitive plural noun trjáa, it takes the strong genitive plural ending -ra. Adding -ra to hár gives you hárra.

Is it always á milli, or can I just say milli?

You can actually just say milli. Both á milli and milli mean "between", and both require the following words to be in the genitive case. You will hear native speakers use them interchangeably.