Þegar spurningarorðið er skýrt, verður öll setningin auðveldari að skilja.

Þegar means when here, and it introduces a subordinate clause:

Þegar spurningarorðið er skýrt

That whole part means something like when the question word is clear.

A useful thing to notice is that the clause after Þegar is not the main clause of the sentence. The main clause comes after the comma:

verður öll setningin auðveldari að skilja

So the sentence is built as:

When X is clear, Y becomes easier to understand.

This is because Icelandic normally puts the main verb in the second position of a main clause.

Here, the whole Þegar-clause comes first:

Þegar spurningarorðið er skýrt

Since that entire clause takes the first slot, the verb of the main clause comes next:

verður

and only then the subject:

öll setningin

So the word order is normal for Icelandic. If you put the main clause first, you would get:

Öll setningin verður auðveldari að skilja þegar spurningarorðið er skýrt.

Here it means becomes.

The verb verða can mean different things depending on context. In many sentences it can have a future sense, but when it is followed by an adjective, it often means become / get.

So:

verður auðveldari = becomes easier

Compare:

• er auðveldari = is easier
• verður auðveldari = becomes easier

In Icelandic, the definite article is often attached to the end of the noun instead of appearing as a separate word.

So:

• orð = word
• orðið = the word

and:

• setning = sentence
• setningin = the sentence

That means:

• spurningarorðið = the question word
• setningin = the sentence

This attached article is very common in Icelandic.

It is a compound noun.

The main parts are:

• spurning = question
• orð = word

Together they make spurningarorð, meaning question word or interrogative word.

Then the definite article is added at the end:

• spurningarorð = question word
• spurningarorðið = the question word

Icelandic uses compounds very heavily, so long words like this are extremely common.

Because skýrt has to agree with spurningarorðið.

Here, skýrt is a predicate adjective after er, and in Icelandic predicate adjectives agree with the noun they describe in gender and number.

spurningarorðið is neuter singular, so the adjective also appears in the neuter singular form:

• skýr = masculine form
• skýr = feminine form
• skýrt = neuter form

So:

spurningarorðið er skýrt = the question word is clear

Because öll has to agree with setningin.

The noun setning is feminine, and here it is singular and nominative. The word allur changes form to match the noun, and the correct form here is öll.

So:

• allur = all / whole
• öll setningin = the whole sentence

This may feel slightly backwards to an English speaker, because Icelandic says something structurally closer to whole the sentence, but the natural English meaning is the whole sentence.

Auðveldari is the comparative form of auðveldur, so it means easier.

Base form:

• auðveldur = easy

Comparative:

• auðveldari = easier

In this sentence, the idea is that once the question word is clear, the sentence does not just remain easy; it becomes easier.

So:

verður öll setningin auðveldari = the whole sentence becomes easier

Að skilja is an infinitive phrase meaning to understand.

So:

• auðveldari = easier
• auðveldari að skilja = easier to understand

This is a very common Icelandic pattern:

adjective + að + infinitive

For example:

• auðvelt að lesa = easy to read
• erfitt að finna = hard to find
• gaman að sjá = fun to see

So in your sentence, að skilja explains in what way the sentence is easier.

Both main nouns are in the nominative, because each one is the subject of its own clause.

• spurningarorðið is the subject of er
• öll setningin is the subject of verður

That is why the adjectives agree with them:

• spurningarorðið → skýrt
• setningin → auðveldari

Even though the sentence is long, it really contains two simple subject-verb structures joined together.

These are two very important Icelandic letters:

• þ is like the th in thing
• ð is like the th in this

In this sentence:

• Þegar begins with þ
• spurningarorðið contains ð
• verður also contains ð

So a learner should notice right away that Icelandic distinguishes two different th sounds, much like English does.