Blaðlaukurinn er grænn.

Because Icelandic usually puts the definite article the onto the end of the noun instead of using a separate word.

• blaðlaukur = leek / a leek
• blaðlaukurinn = the leek

Here, -inn is the masculine singular nominative definite ending.

Usually, no separate article is used.

So:

• blaðlaukur can mean a leek or just leek, depending on context.
• blaðlaukurinn specifically means the leek.

This is different from English, where you normally need a/an or the.

Blaðlaukur is masculine.

You can tell here from two clues:

• the definite form blaðlaukurinn ends in -inn, a common masculine nominative singular definite ending
• the adjective is grænn, which is the masculine singular nominative form of green

Also, many Icelandic nouns ending in -ur are masculine, though that is not a perfect rule.

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

Here, blaðlaukurinn is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative:

• masculine: grænn
• feminine: græn
• neuter: grænt

That is why the sentence uses grænn.

Because this adjective is being used predicatively, not attributively.

In Blaðlaukurinn er grænn, the structure is:

• subject: Blaðlaukurinn
• verb: er
• predicate adjective: grænn

After er (is), Icelandic normally uses the strong adjective form, so you get grænn.

But when the adjective comes directly before a definite noun, you usually use the weak form:

• græni blaðlaukurinn = the green leek

So:

• Blaðlaukurinn er grænn = The leek is green
• græni blaðlaukurinn = the green leek

It is in the nominative case because it is the subject of the sentence.

In a basic sentence with to be:

• the subject is usually nominative
• the predicate adjective agrees with it

So both the noun and the adjective are in the nominative here:

• blaðlaukurinn
• grænn

Er means is. It is the present tense singular form of the verb að vera (to be).

A few useful forms are:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is
• við erum = we are

So Blaðlaukurinn er grænn follows the very familiar pattern:

subject + is + adjective

Yes. This is the normal basic order:

Subject + Verb + Complement

So:

• Blaðlaukurinn = subject
• er = verb
• grænn = complement / predicate adjective

This is very similar to English:

• The leek is green

Icelandic can change word order in some contexts, especially in questions or for emphasis, but this sentence is completely standard.

A rough guide is:

BLAATH-löy-kur-inn

A few helpful points:

• stress is usually on the first syllable in Icelandic
• ð is like the th in this
• au is pronounced somewhat like öy/oy
• -inn is pronounced as a separate ending

This is only an approximation, but it is enough to get started.

A rough approximation is:

grightn or grine-n with a short clipped ending

The exact Icelandic sound is not exactly the same as any single English vowel, but the important points are:

• gr is pronounced together at the start
• æ is a distinct Icelandic vowel sound
• the -nn is part of the adjective ending, not a separate word

If you are learning pronunciation, it is best to hear this from native audio, because Icelandic vowels and consonants often do not match English very closely.

No. Icelandic changes the adjective form depending on how it is used.

• Blaðlaukurinn er grænn = The leek is green
• grænn blaðlaukur = a green leek
• græni blaðlaukurinn = the green leek

So the form of the adjective depends on whether it comes:

• after er as a predicate adjective
• before the noun as an attributive adjective
• with a definite or indefinite noun

That is a very common thing for learners to notice in Icelandic.