Learn Hardcore Icelandic: Ég tala aðeins íslensku hér.

In this sentence, most readers will take aðeins to mean only (restricting what is spoken to Icelandic). However, aðeins can also mean a little / slightly, so the string Ég tala aðeins íslensku can be ambiguous out of context. To say “a little” unambiguously, prefer Ég tala smá íslensku or Ég kann aðeins íslensku. For “only,” you can also use the very common bara or the formal einungis: Ég tala bara/einungis íslensku hér.

Why is it íslensku and not íslenska?

Because of case. Íslenska is a feminine noun (“Icelandic, the language”). After tala when you name a language, the language noun is in the accusative singular, which for this noun is íslensku. Minimal pattern:

Nom. sg.: íslenska (e.g., Íslenskan er falleg. = Icelandic is beautiful.)
Acc./Dat./Gen. sg.: íslensku (e.g., Ég tala íslensku.)

Could I say Ég talar?

No. Verbs agree with the subject. Present tense of tala:

ég tala
þú talar
hann/hún/það talar
við tölum
þið talið
þeir/þær/þau tala

Where should aðeins go, and does its position change the meaning?

By default, put sentence adverbs like aðeins after the finite verb: Ég tala aðeins … To make “only” specifically modify a particular phrase, place it close to that phrase:

Only the language: Ég tala aðeins íslensku hér.
Only the place (not elsewhere): more natural as Ég tala íslensku bara hér. You can front for emphasis too: Aðeins hér tala ég íslensku. The finite verb stays in 2nd position in main clauses.

Why isn’t there a word for “in” before the language (like “in Icelandic”)?

With speaking, Icelandic typically says tala [language in acc.]: Ég tala íslensku. You will also hear tala á íslensku (“speak in Icelandic”), which is common in speech; both are understood, but the bare accusative is the classic, concise pattern. With other verbs, the preposition is normal: skrifa á íslensku, syngja á íslensku.

What’s the difference between hér, hérna, þarna, and þar?

hér: here (at/within the place where I am); neutral/standard.
hérna: “over here,” more colloquial/demonstrative.
þarna: there (visible/identifiable location).
þar: there (general/less immediate, not necessarily visible). Your sentence uses hér because it means “here (in this place).”

Can I move hér to the front?

Yes: Hér tala ég aðeins íslensku. This fronting emphasizes the location. Note the verb is still in 2nd position (Icelandic is a V2 language).

Do Icelandic and the pronoun “I” get capitalized like in English?

No. Language names and the first-person pronoun are lowercase in the middle of sentences: íslenska, ég. They’re capitalized only at the beginning of a sentence or for normal proper-noun rules (e.g., Íslenska at sentence start).

How do I pronounce the sentence?

Approximate guide:

Ég ≈ “yeh” (the g is a soft sound and often very light)
tala ≈ “TAH-lah”
aðeins ≈ “A-th-ains” (the ð is like the th in “this”)
íslensku ≈ “EES-lens-ku” (the final u is like German ü/short French u)
hér ≈ “hyair” (light y-sound, trilled r) Natural stress is on the first syllable of each word.

Is there a better everyday word than aðeins for “only”?

Yes, bara is the most common colloquial choice: Ég tala bara íslensku hér. The formal alternative is einungis. All three (aðeins/bara/einungis) can mean “only”; register and style differ.

Could I say íslenskt instead of íslensku?

No. Íslenskt is an adjective form (“Icelandic” as a descriptor of a neuter noun), not the noun for the language. When you mean the language itself, use the noun íslenska (in the right case: here, íslensku).

Do I need an article (“the Icelandic”)?

No. Icelandic has no indefinite article, and you normally don’t use a definite article with language objects after tala. Use the bare noun in the right case: Ég tala íslensku. You do see the definite form for generic statements as subjects: Íslenskan er falleg.

How would I say “I speak Icelandic only here (not elsewhere)” unambiguously?

Use a placement that clearly targets the location: Ég tala íslensku bara hér. You can also front the location: Aðeins hér tala ég íslensku.