Learn Hardcore Icelandic: Undirskriftin er skýr.

Undirskrift-in = signature + the; the subject (feminine nominative singular). Built from undir (under) + skrift (writing) with the definite ending -in.
er = is; 3rd person singular of vera (to be); the copula.
skýr = clear; a predicate adjective describing the subject.

Why does the noun end in -in? Where is “the”?

Icelandic uses a suffixed definite article. For feminine nouns in nominative singular, the ending is -in. So undirskrift → undirskriftin = “the signature.” (For reference: masculine -inn, feminine -in, neuter -ið.)

What gender/case/number is undirskriftin, and why?

It’s feminine nominative singular. It’s the subject of the sentence, and subjects of the copula vera (“to be”) are in the nominative. The final -t in undirskrift is part of the root; it doesn’t make the noun neuter.

Why is the adjective skýr and not skýrt or skýra?

Predicate adjectives agree with the subject. With a feminine singular subject (undirskriftin), the form is skýr. (For this adjective, masculine and feminine nominative singular look the same.)
skýrt is the neuter form (also the adverb “clearly”: Hann skrifar skýrt = He writes clearly).
skýra is the weak feminine form used attributively with a definite noun: skýra undirskriftin = the clear signature.

How would I say “The signatures are clear”?

Undirskriftirnar eru skýrar.

undirskriftirnar = the signatures (feminine nominative plural, definite)
eru = are
skýrar = clear (feminine nominative plural)

How do I negate the sentence?

Place ekki after the verb: Undirskriftin er ekki skýr.
You can also choose a different adjective:

Undirskriftin er óskýr. = The signature is unclear.
Undirskriftin er ólæsileg. = The signature is illegible.

How do I turn it into a yes/no question and answer it?

Invert verb and subject: Er undirskriftin skýr?
Answers:

Já, hún er skýr. (Yes, it is clear.) [hún because the noun is feminine]
Nei, hún er ekki skýr.

Can I change the word order for emphasis?

Yes. Fronting the adjective is possible for emphasis or style: Skýr er undirskriftin. This is marked/poetic; the neutral order is Undirskriftin er skýr.

How do I say “a clear signature” vs “the clear signature”?

a clear signature: skýr undirskrift (adjective before an indefinite noun; strong form)
the clear signature: skýra undirskriftin or hin skýra undirskrift (with a definite noun; weak form skýra)

Which pronoun replaces undirskriftin?

Use the feminine pronoun hún: Hún er skýr.

How do I pronounce it?

Approximate guide:

Undirskriftin: “UN-dir-skrih-ftin” (u like “put”; r tapped; nd often sounds like nt)
er: “ehr”
skýr: “skeer” (ý = long “ee”; r tapped/rolled) Stress the first syllable: UN-dir-skriftin ehr SKEER.

How does the adjective change with different subjects in this pattern?

Predicate adjectives agree with the subject:

Masculine: Samningurinn er skýr. (The contract is clear.)
Feminine: Undirskriftin er skýr.
Neuter: Skjalið er skýrt.
Plural feminine: Undirskriftirnar eru skýrar.

Is there a difference between undirskrift and undirritun?

Yes:

undirskrift = a signature (the written name/mark)
undirritun = the act of signing (e.g., a signing ceremony) Example: Undirritun samningsins fór fram í morgun og undirskriftin er skýr.

Can I intensify the adjective?

Yes: Undirskriftin er mjög skýr. = The signature is very clear. Other intensifiers: afar, býsna, rosalega (colloquial).

What are the key forms of “vera” (to be) I might need?

Present: ég er, þú ert, hann/hún/það er, við erum, þið eruð, þeir/þær/þau eru
Past: ég var, þú varst, hann/hún/það var, við vorum, þið voruð, þeir/þær/þau voru