Learn Hardcore Icelandic: Tréð fyrir framan húsið er mjög hátt. - The tree in front of the house is very tall.

So instead of saying something like *hið tré, Icelandic normally just sticks the article onto the noun: tréð. (There is a separate form hið, but it’s used in specific, more formal/technical contexts, not in everyday phrases like this.)

Why is it tréð but húsið? Why do we get -ð in one case and -ið in the other?

Both tré and hús are neuter nouns, but they belong to different declension patterns.

tré → tréð (the tree)
hús → húsið (the house)

The exact ending you get (-ð, -ið, etc.) depends on which pattern the noun belongs to. For a learner, the practical rule is:

Learn the definite singular form together with the noun (dictionaries usually list it).
Accept that some neuter nouns take -ð and others take -ið; it’s not predictable from English.

A small mini-paradigm:

tré – tréð – tré (pl.) – tréin (the trees)
hús – húsið – hús (pl.) – húsin (the houses)

So the sentence uses the correct definite forms for each noun, according to their pattern.

What grammatical roles do tréð and húsið have in this sentence?

Tréð is the subject of the sentence. It is in the nominative case:
- Tréð … er mjög hátt. → The tree … is very tall.
Húsið is the object of the preposition fyrir framan (in front of).
Prepositions in Icelandic govern cases; fyrir framan always takes the accusative.
- Here: húsið is accusative singular neuter.

Note: for neuter nouns like hús, nominative and accusative singular look the same (húsið), so you know it’s accusative because of the preposition, not from the form alone.

What exactly does fyrir framan mean, and why does it take the accusative?

fyrir framan is a compound preposition meaning “in front of”. It behaves as a single unit:

fyrir framan húsið = in front of the house

It always uses the accusative case for the noun that follows (húsið, bílinn, húsið o.s.frv.), regardless of whether it’s talking about motion or just location. Other similar expressions also take accusative:

fyrir aftan = behind
fyrir ofan = above
fyrir neðan = below

So you say:

fyrir framan húsið, never fyrir framan húsinu in this meaning.

Why is the adjective hátt and not something like hár or há?

The base adjective here is hár = tall, high. Icelandic adjectives agree with the noun in:

gender
number
case
and whether the noun is definite/indefinite (strong vs. weak declension)

In this sentence:

tréð is neuter singular nominative, definite.
But the adjective is used predicatively after to be (er mjög hátt), and predicative adjectives take the strong form.

Strong nominative singular forms of hár:

masculine: hár
feminine: há
neuter: hátt

Since tréð is neuter, we must use the neuter form:

Tréð … er hátt. → The tree is tall.

Adding mjög:

Tréð … er mjög hátt. → The tree is very tall.

Does hátt here mean “loud” (as in speaking loudly), or “tall/high”? It looks like an adverb too.

Good observation; the form hátt can be:

Adjective, neuter nominative/accusative singular of hár = high, tall
- Used here: Tréð … er mjög hátt. → The tree is very tall.
Adverb from hár = loudly or high (in pitch/volume)
- Example: Hann syngur hátt. → He sings loudly / in a high voice.

In your sentence, hátt is clearly an adjective describing the tree’s height, not the volume of sound. The clue is that it directly agrees with a neuter noun (tréð), instead of modifying a verb like sing, speak, etc.

What is mjög, and where does it go in the sentence?

mjög is an adverb meaning “very”. It:

modifies adjectives and other adverbs
does not change form; it’s always mjög

Word order:

It is placed directly before the adjective or adverb it modifies:
- mjög hátt = very tall
- mjög fallegt = very beautiful
- mjög vel = very well

So in the sentence:

er mjög hátt = is very tall

Why is the word order “Tréð fyrir framan húsið er mjög hátt” and not “Tréð er mjög hátt fyrir framan húsið”?

In Tréð fyrir framan húsið, the phrase fyrir framan húsið belongs tightly to tréð and tells you which tree:

Tréð fyrir framan húsið = the tree in front of the house (identifying which tree)

So the natural structure is:

[Tréð fyrir framan húsið] er mjög hátt.
The tree in front of the house is very tall.

If you say:

Tréð er mjög hátt fyrir framan húsið,

it sounds more like:

The tree is very tall in front of the house,

which is awkward and could be interpreted as describing where it is tall, not which tree you mean. The first version is the normal, clear way to say it.

Could I also say “Tréð sem er fyrir framan húsið er mjög hátt”? What’s the difference?

Yes, that is also grammatical:

Tréð sem er fyrir framan húsið er mjög hátt.
= The tree that is in front of the house is very tall.

Difference:

Tréð fyrir framan húsið…
works like English “the tree in front of the house …” – a noun followed by a prepositional phrase.
Tréð sem er fyrir framan húsið…
corresponds to “the tree that is in front of the house …” – an explicit relative clause with sem (that/which).

Both are natural; the version without sem er is a bit shorter and more like everyday English “the tree in front of the house”.

What gender is tréð, and how do I know that from the form?

tréð is neuter.

Signs:

The base form tré is a typical neuter pattern (many neuter nouns have the same form in nominative sg. and plural).
The definite ending -ð with this stem is characteristic of neuter nouns of this type.
The adjective agreeing with it is neuter: hátt (not masculine hár or feminine há).

In practice, you usually learn the noun with its gender (dictionaries mark it), e.g.:

tré, -ið (n.) → tells you it is neuter and that the definite sg. form is tréð (sometimes written tréð / tréið in dictionaries).

How do you pronounce tréð and húsið, especially the ð at the end?

Rough approximations for an English speaker:

tréð
- tré sounds roughly like tray but shorter and with a bit of [j]-like quality: treh-y.
- ð is the voiced th sound /ð/ as in this, but at the end of the word it’s very soft, almost like the sound is fading out.
- Altogether something like: trehð (soft final th).
húsið
- hú similar to hoo but with a very pure long u sound.
- si like see.
- ð again a soft th sound at the end.
- Altogether: HOO-see-ð (soft final th).

In both words, the ð is never pronounced like English d; it’s always a (usually quite weak) voiced th sound.