Learn Hardcore Icelandic: Ég var mjög þreyttur í gær.

If the speaker is male:
Ég var mjög þreyttur í gær. – I (male) was very tired yesterday.
If the speaker is female:
Ég var mjög þreytt í gær. – I (female) was very tired yesterday.
(Here þreytt is the feminine singular nominative form.)

So the form þreyttur is used because the implied speaker is grammatically masculine. The rest of the sentence stays the same.

Does Ég itself have gender, or how does the adjective know whether to be þreyttur or þreytt?

The pronoun Ég (I) does not change form for gender; it is always Ég.

The adjective’s form (þreyttur, þreytt, etc.) is chosen according to the natural gender of the speaker:

Man speaking: Ég er/var þreyttur.
Woman speaking: Ég er/var þreytt.
Non‑binary people often choose one form or may use other strategies, but standard grammar assumes masculine or feminine agreement.

So the grammar assumes:
Ég (speaker is male) → adjective in masculine form (þreyttur)
Ég (speaker is female) → adjective in feminine form (þreytt)

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

Mjög means very. It’s an adverb that usually comes directly before the adjective or adverb it modifies:

mjög þreyttur – very tired
mjög góður – very good
mjög seint – very late

Typical word order is:

[subject] + [verb] + [mjög] + [adjective] + [other elements]

So:

Ég var mjög þreyttur í gær. – I was very tired yesterday.

You normally would not move mjög away from þreyttur; they stick together as a unit.

What does í gær literally mean, and why do we need í?

Í gær means yesterday, but literally it’s like saying in yesterday.

í – usually in, on, or at (a general preposition)
gær – yesterday (this word almost always appears with í)

Time expressions with í are very common in Icelandic:

í gær – yesterday
í dag – today
í fyrradag – the day before yesterday

You cannot normally say just gær on its own as a time adverb; you say í gær.

What case is used in í gær, and does í always take that case?

Formally, gær is treated as being in the accusative after í in this time expression.

The preposition í can take either accusative or dative, depending on meaning:

accusative: movement into / through / over time
- Ég fer í skólann. – I am going to school.
- Í gær – in/over the period of yesterday.
dative: location / being in something
- Ég er í skólanum. – I am at school.

With set time phrases like í gær and í dag, you just learn them as fixed expressions; they use the accusative form.

Can I change the word order, for example: Í gær var ég mjög þreyttur?

Yes. Both are correct, but they differ slightly in emphasis:

Ég var mjög þreyttur í gær.
Neutral, default order. Focuses first on I was very tired, then adds yesterday.
Í gær var ég mjög þreyttur.
Brings yesterday to the front, often used if you’re contrasting days or emphasizing yesterday as the time frame.

The important rule is that in a main clause the finite verb (var) must be in second position:

1st slot: Í gær (adverbial phrase)
2nd slot: var (verb)
then: ég mjög þreyttur

How would I say I am very tired instead of I was very tired?

Just change the verb from past to present:

Ég er mjög þreyttur. – I (male) am very tired.
Ég er mjög þreytt. – I (female) am very tired.

So:

er = am / is / are (present)
var = was (past)

Is there a way to say I have been very tired in Icelandic, like a present perfect?

Yes, you can use the present perfect:

Ég hef verið mjög þreyttur. – I (male) have been very tired.
Ég hef verið mjög þreytt. – I (female) have been very tired.

Breakdown:

hef – auxiliary verb have (for ég)
verið – past participle of vera (been)
mjög þreyttur / þreytt – very tired

This suggests tiredness over a period up to now, similar to English have been.

How do you pronounce Ég var mjög þreyttur í gær?

Approximate pronunciation (IPA) and tips:

Ég – [jɛiː]
- The g is not a hard g; it’s more like a y sound.
var – [var]
- Like var in varnish, short a.
mjög – [mjœːɣ] or [mjœːɡ] (depending on speaker)
- mj together: like English my but with a clear y.
- ö (œ) is like the vowel in French peu.
- Final g/ɣ is a soft consonant, often not strongly released.
þreyttur – [θreihtʏr] (approx.)
- þ = voiceless th, like th in think.
- ey = like ei in eight or hey.
- Double tt makes a harder, longer t sound.
- u in -tur is a short, rounded vowel, similar to the u in German Mutter.
í – [iː]
- Like ee in see, long vowel.
gær – [jaiːr] or [gaiːr] (variation)
- Often sounds like y-ire, similar to guy but with a bit of air at the end.

Said at normal speed, it flows roughly like:

Yei var myöɣ θreittur í yair (for a very rough English-like approximation).

Are there plural or other-person versions of this sentence that are worth knowing?

Yes, it’s good to see the pattern with different subjects. Using þreyttur for a masculine group and þreyttar for a feminine plural:

Þú varst mjög þreyttur í gær. – You (male, sing.) were very tired yesterday.
Þú varst mjög þreytt í gær. – You (female, sing.) were very tired yesterday.
Við vorum mjög þreyttir í gær. – We (all or mostly male / mixed) were very tired yesterday.
Við vorum mjög þreyttar í gær. – We (all female) were very tired yesterday.
Þeir voru mjög þreyttir í gær. – They (masc.) were very tired yesterday.
Þær voru mjög þreyttar í gær. – They (fem.) were very tired yesterday.
Þau voru mjög þreytt í gær. – They (neut.; mixed or children) were very tired yesterday.

The structure remains the same; only the verb form and the adjective ending change to agree with the subject.