Leiguskilmálarnir eru skýrir.

Because Icelandic very often makes compound nouns where English would use two words.

Leiguskilmálarnir breaks down like this:

• leigu- = rental / of rent
• skilmálar = terms, conditions
• -nir = the for a masculine plural noun in the nominative

So the whole word means the rental terms or the lease conditions.

-nir is the definite article attached to the noun. In Icelandic, the is usually added to the end of the noun instead of written as a separate word.

So:

• leiguskilmálar = rental terms
• leiguskilmálarnir = the rental terms

This is very normal in Icelandic.

Because the subject is plural.

The verb að vera means to be. In the present tense:

• er = is
• eru = are

Since leiguskilmálarnir means the rental terms, you need the plural verb:

• Leiguskilmálarnir eru skýrir. = The rental terms are clear.

Because Icelandic adjectives must agree with the noun they describe in:

• gender
• number
• case

Here, leiguskilmálarnir is:

• masculine
• plural
• nominative

So the adjective must also be masculine plural nominative:

• skýr = clear, explicit
• skýrir = clear (masculine plural nominative)

That is why you get:

• Leiguskilmálarnir eru skýrir.

That is a very good question. In Icelandic, predicate adjectives after verbs like vera usually stay in the strong form, even when the noun is definite.

So:

• Leiguskilmálarnir eru skýrir. = correct

Even though leiguskilmálarnir is the rental terms, the adjective after eru is still skýrir, not a weak form.

This is different from an adjective placed directly before a noun, where weak forms are more likely in definite phrases.

The subject leiguskilmálarnir is in the nominative plural. Since the sentence uses vera and the adjective describes the subject, skýrir is also in the nominative plural.

So both main parts line up grammatically:

• leiguskilmálarnir = nominative plural masculine definite
• skýrir = nominative plural masculine

Because here skýrir is a predicate adjective, not an adjective directly attached to the noun.

Compare:

• Leiguskilmálarnir eru skýrir. = The rental terms are clear.
• skýrir leiguskilmálar = clear rental terms

In the sentence you were given, the structure is:

• subject + verb + predicate adjective

So the adjective naturally comes after eru.

Yes. The singular is skilmáli, meaning a term or a condition.

So you could also have:

• Leiguskilmálinn er skýr. = The rental term is clear.

But in real usage, words like skilmálar are very often used in the plural, much like English terms and conditions.

Here skýr means something like:

• clear
• easy to understand
• explicit
• unambiguous

With something like rental terms, it usually means the terms are clearly stated or easy to understand, not that they are physically clear in a visual sense.

You would remove the definite ending -nir:

• Leiguskilmálar eru skýrir. = Rental terms are clear.

Compare:

• Leiguskilmálar = rental terms
• Leiguskilmálarnir = the rental terms

So the difference is mainly whether you mean rental terms in general / some rental terms or the specific rental terms.

A rough learner-friendly guide would be:

• Leiguskilmálarnir ≈ LAY-gu-skil-MOW-la-nir
• eru ≈ EH-ru
• skýrir ≈ SKEE-rir

A few helpful points:

• Icelandic stress usually falls on the first syllable of each word.
• ý sounds roughly like ee in see.
• The r sound is usually tapped or rolled more than in English.

So the rhythm is roughly:

LAY-gu-skil-MOW-la-nir EH-ru SKEE-rir