Learn Hardcore Icelandic: Heimilið er hreint og hlýtt.

Heimili is a neuter noun. That matters because adjectives must agree with the noun in gender, number, and case. Since the subject is neuter singular nominative, the predicate adjectives are also neuter singular nominative: hreint, hlýtt.

Why do the adjectives end in -t (hreint, hlýtt)?

Predicate adjectives after “to be” (vera) agree with the subject. The neuter singular nominative ending for strong adjectives is -t, hence hreint (from hreinn) and hlýtt (from hlýr).

Why is it hreinT but hlýTT? Why two different spellings?

It’s due to regular sound/spelling patterns when forming the neuter:

hreinn → neuter hreint (the sequence ends up as -nt)
hlýr → neuter hlýtt (adjectives ending in -r typically make neuter -tt) Common parallels: góður → gott, nýr → nýtt, gamall → gamalt.

Could I say Heimilið er hreinn og hlýr instead?

No. Hreinn and hlýr are masculine forms. Because heimilið is neuter, you must use hreint and hlýtt. Using masculine forms here is ungrammatical.

When would I use the weak forms of these adjectives?

Use weak adjective endings when an adjective directly modifies a definite noun (or with certain determiners). For example:

hreina og hlýja heimilið = “the clean and warm home” (adjectives before a definite noun take weak endings) You may also see the separate article before the adjective in more formal style: hið hreina og hlýja heimili.

What does og mean and how is it pronounced?

Og means “and.” In careful speech it’s like a short “oh” with a soft/weak final consonant; in everyday speech the g often disappears, so it often sounds close to just “oh.”

How do I pronounce the words here?

Heimilið: stress the first syllable; ei like “ay” in “say”; final ð is the voiced “th” in “this” (often very soft). Approx: “HAY-mi-lith.”
hreint: initial hr- has a voiceless r; ei like “ay.” Approx: “hraynt.”
hlýtt: initial hl- has a voiceless l; ý like a long “ee”; tt is preaspirated (a little breath before t). Approx: “hlee-ht.”
General rule: stress almost always on the first syllable in Icelandic.

Are í and ý different sounds?

In modern Icelandic, í and ý are pronounced the same (a high front vowel like English “ee”). They differ in spelling for historical/etymological reasons, not pronunciation.

What’s the difference between hlýtt and heitt?

Both relate to temperature, but:

hlýtt = comfortably warm, cozy (a pleasant warmth)
heitt = hot/warm (often “hot” to the touch or high temperature) So a room is typically hlýtt; boiling water is heitt.

Could I use hús instead of heimili?

Yes, but the meaning shifts slightly:

Heimilið = the home/household (the place you live; more “homey”)
Húsið = the house (the building) Grammar stays the same: Húsið er hreint og hlýtt.

How would I say “My home is clean and warm”?

Heimilið mitt er hreint og hlýtt.
Mitt heimili er hreint og hlýtt. The possessive mitt (neuter singular nominative) agrees with heimilið. Postposing (heimilið mitt) is very common.

How do I make it plural: “The homes are clean and warm”?

Heimilin eru hrein og hlý. Notes:
Definite plural of heimili is heimilin.
Plural “are” is eru (from vera).
Neuter plural strong adjectives have no -t: hrein, hlý.

How do I negate this sentence?

General negation: Heimilið er ekki hreint og hlýtt. (It isn’t clean and warm.)
“Neither … nor …”: Heimilið er hvorki hreint né hlýtt. (Stronger, excludes both qualities.) Use ekki for “not,” and the correlative hvorki … né … for “neither … nor ….”

What’s the verb here, and how is it conjugated?

The verb is er (“is”), from vera (“to be”). Present tense:

ég er
þú ert
hann/hún/það er
við erum
þið eruð
þeir/þær/þau eru

Can I change the word order?

Yes, Icelandic allows fronting, but keeps the verb in second position (V2). For emphasis you could say:

Hreint og hlýtt er heimilið. Neutral, most common order is what you saw: Heimilið er hreint og hlýtt.