Þvottahúsið er niðri við hliðina á hjólageymslunni.

Because -ið is the definite article for this noun in the neuter singular nominative.

• þvottahús = laundry room / wash house
• þvottahúsið = the laundry room

In Icelandic, the word for the is usually attached to the end of the noun instead of being a separate word.

Þvottahús is a neuter noun. Gender matters because it affects:

• the form of the definite article
• adjective agreement
• sometimes other endings in the sentence

Here, the neuter nominative singular definite form is þvottahúsið.

Niðri means downstairs, down below, or down there, depending on context.

In this sentence, niðri most naturally means downstairs:

• Þvottahúsið er niðri... = The laundry room is downstairs...

It describes location, not motion.

This is a very common question.

• niðri = down / downstairs as a location
• niður = down as movement toward a lower place

So:

• Þvottahúsið er niðri. = The laundry room is downstairs.
• Ég fer niður. = I am going downstairs.

A useful shortcut is:

• Where is it? → often niðri
• Where is it going? → often niður

Við hliðina á is a set phrase meaning next to, beside, or more literally at the side of.

You should learn it as one unit:

• við hliðina á skólanum = next to the school
• við hliðina á mér = next to me

Even though the phrase contains several parts, learners usually remember it best as a single expression.

Because the full expression is við hliðina á. The á is part of the idiom.

Literally, the phrase is built something like:

• við = by / at
• hliðina = the side
• á = of / on / at, depending on context

But you do not need to translate each piece separately every time. It is best to think of við hliðina á simply as next to.

Because after við hliðina á, the following noun goes in the dative case, and here it is also definite.

Base noun:

• hjólageymsla = bike storage room / bicycle storage

In this sentence it becomes:

• hjólageymslunni = the bike storage room in the dative singular definite

So the ending changes because of case and definiteness.

The clue is the phrase við hliðina á, which takes the dative.

So:

• hjólageymsla = indefinite base form
• hjólageymslunni = dative singular definite

This is very common in Icelandic: certain prepositions or fixed prepositional phrases require a specific case.

It is a compound noun, which is extremely common in Icelandic.

It is made from:

• hjóla- = from hjól, meaning bicycle / wheel
• geymsla = storage, storeroom

So hjólageymsla means something like bicycle storage room or bike storage area.

Icelandic often combines nouns this way instead of using a longer phrase.

Because Icelandic often puts location expressions after the verb er.

So the structure is:

• Þvottahúsið = subject
• er = is
• niðri við hliðina á hjólageymslunni = location

This is very natural Icelandic word order for saying where something is.

A very literal layout would be:

• The laundry room
  • is
    • downstairs next to the bike storage room

Yes, if you only want to say that the laundry room is next to the bike storage room.

• Þvottahúsið er við hliðina á hjólageymslunni. = The laundry room is next to the bike storage room.

Adding niðri gives extra information: it tells you that this location is downstairs/down below.

Most naturally, something like:

• The laundry room is downstairs next to the bike storage room.

That is smoother in English than a word-for-word version such as:

• The laundry room is down by the side of the bike storage room.

So the Icelandic is quite normal and idiomatic.

A rough guide for an English speaker:

• Þ sounds like th in thin
• votta is roughly vot-ta
• hús is like hoos
• ið ends with a soft th sound, like th in this, though actual Icelandic pronunciation can be subtler

A rough approximation is:

• THVOT-ta-hoo-sith

But listen to native audio if possible, because Icelandic sounds are hard to learn well from spelling alone.

These are two important Icelandic letters:

• Þ þ = unvoiced th, like th in thin
• Ð ð = voiced th, like th in this

In this sentence:

• Þ appears in Þvottahúsið
• ð appears in hliðina

English used to have these letters long ago, but modern English no longer uses them.

Yes. It comes from hlið, meaning side.

In the phrase við hliðina á, it appears in a fixed form that is part of the idiom next to.

So while hliðina is historically and grammatically a noun form, learners usually benefit most from treating the full phrase við hliðina á as one expression.

It is:

• [subject] + er + [location]

So here:

• Þvottahúsið = subject
• er = present tense of to be
• niðri við hliðina á hjólageymslunni = location phrase

This is a very useful pattern for beginners, because you can reuse it easily:

• Eldhúsið er uppi. = The kitchen is upstairs.
• Bíllinn er fyrir utan húsið. = The car is outside the house.
• Herbergið er við hliðina á baðherberginu. = The room is next to the bathroom.