Stóllinn er lægri en borðið.

Use en after a comparative adjective (the -ri form) to mean “than.”

• Comparative: Stóllinn er lægri en borðið.
• Equality uses jafn … og/sem: Stóllinn er jafn lágur og/sem borðið (“as low as”).
• Preference uses words like frekar en / fremur en (“rather than”) and contrast uses en = “but”: Hann er lágur, en hún er há.

Because it’s a comparison. The base adjective is lágur (“low”), but the comparative is lægri (“lower”). It must agree with the subject:

• Masculine subject: Stóllinn er lægri…
• Neuter subject: Borðið er lægra…
  lágt is the neuter positive form (“low”), not the comparative.

Yes. That’s correct and shows agreement with the subject:

• Borðið (neuter) → lægra
• Stóllinn (masculine) → lægri
  The adjective agrees with the subject before the verb, not with the noun after en.

There’s a vowel change (umlaut) in the stem: lágur → lægri (comparative) → lægstur (superlative). Other common patterns:

• hár (“tall/high”) → hærri → hæstur
• stór (“big”) → stærri → stærstur
• Fully irregular: góður → betri → bestur, mikill → meiri → mestur

After en, the compared element takes the case it would have in the “understood” clause. Here it’s like saying “the chair is lower than the table is,” so nominative: borðið, not dative borðinu.
With pronouns, standard Icelandic prefers nominative when it’s the understood subject:

• Ég er hærri en þú (ert), though colloquial speech often uses object forms (en þig).
  If the understood verb would assign another case, you use that:
• Ég þekki hann betur en hana (þekki).

They’re definite because you’re talking about specific, identifiable items (“the chair,” “the table”).

• Specific: Stóllinn er lægri en borðið.
• Generic statement: Stóll er lægri en borð (“A chair is lower than a table”).
  Note: Icelandic has no separate word for “a/an”; indefiniteness is just the bare noun.

Because of grammatical gender:

• stóll (masculine) + definite = stóllinn
• borð (neuter) + definite = borðið
  Icelandic adds the definite article as a suffix that matches the noun’s gender/number/case.

Yes, en can mean both “than” and “but.” Context disambiguates:

• “than”: Stóllinn er lægri en borðið.
• “but”: Stóllinn er lágur, en borðið er hátt.
  Don’t confuse en with enn (“still/yet”): Hann er enn lægri (“He’s still lower”).

Approximate English-friendly guide:

• ó ≈ long “o” as in “go” (a bit rounded).
• æ ≈ “eye.”
• ll in stóllinn is a voiceless “tl” sound. So: stóllinn ≈ “STOHL-tlin.”
• ð in borðið is the voiced “th” of “this.” The ending -ið ≈ “-ith.” So: borðið ≈ “BOR-thith.”
• en ≈ “en” (like “pen” without the p).
  This is only a rough guide; real Icelandic has length and devoicing patterns you’ll pick up with listening.

Use lágur for “low” (vertical height/position). stuttur means “short” (length/duration), e.g., a short cable or a short meeting. For tall/high, use hár (adj.):

• Stóllinn er lágur / lægri.
• Borðið er hátt / hærra.
  For people, lágvaxinn (“short of stature”) is also common.

These express preference/contrast, not a plain comparative of an adjective:

• Preference: Ég vil te frekar en kaffi. (“I’d rather have tea than coffee.”)
• Contrast after negation: Ekki þetta, heldur en hitt.
  For adjective comparisons, you normally just use the comparative + en: lægra en, hærra en, betra en.