The word “the” is known as the definite article. In English it is a seperate word (“the house”) but in Icelandic it is attached to the end of the relevant word (“húsið”).
Notice that there is no “a / an” in Icelandic:
|Mary is an American.
|María er Ameríkani.
|He is such a good dog.
|Hann er svo góður hundur.
|This takes a long time.
|Þetta tekur langan tíma.
And notice that the “the” can look quite different depending on the word it is attached to:
|The weather today is good.
|Veðrið í dag er gott.
|Why is the cat outside?
|Af hverju er kötturinn úti?
|The bakery is open!
|Bakaríið er opið!
Again, the “the” comes at the end of words instead of at the front. The endings in veðrið and kötturinn mean the exact same thing, they just signify “the”. The “the” changed depending on the word’s grammatical type (gender and case), which results in many possible endings. These are all the possible endings for “the”:
:-inn -ið -inum -ins -num -an -una -unni -unnar -nar -unum -anna -inu -ins -in
You do not need to memorize these endings for now, but you do need to be aware of them.