Learn Hardcore Icelandic: Strákurinn er mjög hár.

So -inn here is the masculine singular definite ending in the nominative case.
The sentence is specifically talking about “the boy”, not just “a boy” in general, so it must be strákurinn.

What is the base dictionary form of strákurinn, and what are its gender and case?

The base (dictionary) form is:

strákur – masculine noun, nominative singular, meaning boy.

In the sentence Strákurinn er mjög hár:

strákurinn is:
- masculine
- singular
- nominative
- definite (because of -inn)

The nominative case is used because strákurinn is the subject of the sentence.

How do you pronounce Strákurinn er mjög hár roughly in English-like sounds?

Very roughly (not exact IPA), you can think of it like:

Strákurinn ≈ STRAU-kur-in
- á is like the “ow” in cow (but shorter and tenser)
- the u is a short, rounded sound, somewhat like the u in put
- the final -inn is like -in but with a bit more clear n.
er ≈ air (but shorter)
mjög ≈ myug
- mj is like “my” but with both sounds pronounced together
- ö is a rounded sound like the vowel in French deux or German schön
- final g is often very soft, almost disappearing in normal speech.
hár ≈ howr
- há sounds like how (again, shorter/tenser)
- the r is rolled or tapped.

Altogether: something like “STRAU-kur-in air myug howr”, with all vowels a bit tenser and cleaner than in English.

Why is the word for “tall” hár, and not changed to something like há or hátt?

Icelandic adjectives agree with the noun they describe in gender, number, and case.

For the adjective hár (“tall, high”):

masculine nominative singular: hár
feminine nominative singular: há
neuter nominative singular: hátt

In Strákurinn er mjög hár:

strákurinn is masculine singular nominative
so the adjective in a predicate (after er) also appears in masculine singular nominative
that form is hár.

If the subject were feminine or neuter, you’d see:

Stelpan er mjög há. – The girl is very tall.
Húsið er mjög hátt. – The house is very tall/high.

Why is the adjective hár placed after the verb, not before the noun like in English?

This sentence uses a predicative adjective: an adjective that comes after a linking verb (“to be”) and describes the subject.

English:

The boy is tall → adjective after is.

Icelandic works the same here:

Strákurinn (subject)
er (linking verb “is”)
mjög hár (adverb + adjective describing the subject)

If the adjective came before the noun as an attributive adjective, the structure and form would change:

hár strákur = a tall boy
hái strákurinn = the tall boy (adjective changes form because it directly modifies a definite noun)

But in the sentence you gave, it’s a full sentence with “is”, so the adjective naturally comes after the verb.

What does mjög do in this sentence, and where does it normally go?

Mjög is an adverb meaning “very”. It intensifies the adjective that follows.

hár = tall
mjög hár = very tall

Placement:

mjög almost always comes directly before the word it modifies:
- mjög hár – very tall
- mjög stór – very big
- mjög gamall – very old

So Strákurinn er mjög hár literally has the structure:
The-boy is very tall.

Could you say Hár er strákurinn instead, and what would that mean?

You could say Hár er strákurinn, but:

it sounds unusual or poetic in modern everyday Icelandic
it has a nuance of emphasis or a slightly “stylistic” word order.

Normal, neutral word order is:

Strákurinn er hár. – The boy is tall.

When you start with the adjective:

Hár er strákurinn. – Tall is the boy. (something like “Tall, that boy is”; more marked/emphatic)

For an ordinary statement, learners should stick with Strákurinn er (mjög) hár.

Is hár here related to the word for “hair” in Icelandic?

Yes – they look the same in spelling, but function differently.

hár (adjective) = tall, high
- changes form according to gender/number/case: hár, há, hátt, etc.
- used like in Strákurinn er mjög hár – The boy is very tall.
hár (noun, neuter) = a hair / hair
- definite: hárið = the hair
- plural: hár can also be hairs depending on context.

So context and grammar tell you which one it is:

Strákurinn er mjög hár – adjective (tall).
Hárið er blautt – The hair is wet. (noun hair
- adjective wet).

Why is the verb er used here, and does it change with the subject?

Er is the 3rd person singular present form of the verb vera (“to be”).

Present tense of vera:

ég er – I am
þú ert – you (sg.) are
hann / hún / það er – he / she / it is
við erum – we are
þið eruð – you (pl.) are
þeir / þær / þau eru – they are

In Strákurinn er mjög hár:

strákurinn = he / the boy → 3rd person singular
so the correct form is er.

If the subject changed, the verb would change too:

Ég er mjög hár. – I am very tall.
Þeir eru mjög háir. – They (masc.) are very tall.

What case is used for the adjective hár here, and why?

The adjective hár is in the nominative case, masculine singular.

Reason:

The subject strákurinn is nominative masculine singular.
A predicative adjective (one after “to be”) in Icelandic agrees with the subject in gender, number, and case.
So the adjective takes the same case as the subject, which is nominative here.

That’s why we get hár, not another case form like háum, háum, etc.

Can mjög be replaced by some more colloquial words like “really” in English?

Yes, spoken Icelandic often uses more colloquial intensifiers, similar to English really, super, crazy (tall), etc.. Some common ones:

rosa / rosalega – roughly really / insanely
- Strákurinn er rosalega hár.
mjög – neutral, standard very
- Strákurinn er mjög hár.
alveg rosalega hár – really, really tall (more emphatic)

For learners, mjög is the safest, most neutral and correct word to use in all contexts.