Learn Hardcore Icelandic: Buxurnar mínar eru líka svartar.

Because the subject buxurnar (the pants) is grammatically plural. The verb vera (to be) agrees with the subject: singular is er, plural is eru. So: Buxurnar … eru …

What does the -nar ending in buxurnar mean?

It’s the suffixed definite article for feminine plural (nominative/accusative).

buxur = pants (indefinite)
buxurnar = the pants (definite)

Why is the adjective svartar in that form?

Adjectives used as a predicate after “to be” agree in gender, number, and case with the subject and take the strong declension.

buxur is feminine plural nominative
Strong feminine plural nominative of svartur is svartar
Hence: … eru … svartar

Why is it mínar, not mínir or mín?

The possessive must agree with the noun. buxur is feminine plural nominative, so the matching form is mínar.

Masculine plural would be mínir
Neuter plural would be mín

Why is the possessive after the noun (buxurnar mínar) instead of before it?

Postposed possessives are the default and make the noun phrase definite with the suffixed article: buxurnar mínar = “my pants.”
You can put the possessive before the noun (mínar buxur), but that is typically more contrastive or indefinite in feel. For neutral “my X,” Icelandic usually prefers the postposed pattern with the definite suffix.

Could I drop the article and say buxur mínar?

It’s possible but not the usual way to say “my pants.” With a postposed possessive, Icelandic normally keeps the definite suffix: buxurnar mínar. If you want no definite suffix, you’d normally move the possessive before the noun: mínar buxur.

Why isn’t the adjective “made definite”? Shouldn’t it be svörtu?

Definiteness affects attributive adjectives (when they directly modify a noun), not predicative adjectives (after “to be”).

Attributive, definite: (hinar) svörtu buxurnar = “the black pants” (weak declension: svörtu)
Predicative: Buxurnar … eru svartar (strong declension: svartar)

Where does líka (“also”) go? Could I put it somewhere else?

A common placement is after the finite verb: … eru líka svartar.
Other positions are possible for emphasis, e.g., sentence-initial (Líka eru …) or near the end (… eru svartar líka), but … eru líka … is the neutral, most typical choice.

Does líka ever mean “similar”?

Not here. líka in this sentence is the adverb “also/too.” The adjective “similar/likely” is líkur (e.g., Ég er líkur honum = “I am similar to him”), which inflects differently and is not used in this sentence.

Is buxur always plural? How do I say “a pair of pants”?

Yes, buxur is plural-only (pluralia tantum). Basic case forms you’ll see often:

Nom/Acc: buxur
Dat: buxum
Gen: buxna
“A pair of pants” is eitt par af buxum.

How would I say “My pants aren’t black either”?

Use heldur with negation for “either”:

Buxurnar mínar eru ekki svartar heldur.
You can also say: Buxurnar mínar eru heldur ekki svartar. Both are idiomatic.

How do I turn the sentence into a yes–no question?

Invert verb and subject: Eru buxurnar mínar líka svartar?

How do I say “my black pants” as a noun phrase?

As an attributive phrase with a definite noun, use the weak adjective:

(Hinar) svörtu buxurnar mínar = “my black pants”
In everyday speech you’ll most often hear: svörtu buxurnar mínar (the article hinar is usually omitted in conversation).

What would change with a masculine plural noun like “shoes”?

Masculine plural nominative definite takes -nir, and agreement changes accordingly:

Skórnir mínir eru líka svartir.
Here mínir (masc. pl.) and svartir (masc. pl. strong) agree with skórnir (masc. pl. nom. def.).

How do I pronounce the sentence?

Approximate guide (stressed syllables first, Icelandic r is trilled):

Buxurnar ≈ “PÜK-sür-nar”
mínar ≈ “MEE-nar”
eru ≈ “EH-ru”
líka ≈ “LEE-ka”
svartar ≈ “SVAR-tar”
Together: “PÜK-sür-nar MEE-nar EH-ru LEE-ka SVAR-tar.”
(Closer IPA: [ˈpʏksʏrnar ˈmiːnar ˈɛːrʏ ˈliːka ˈsvar̥tar])