Þögnin varð svo vandræðaleg að enginn vissi hvað hann átti að segja.

Þögnin means the silence. The ending -in is the suffixed definite article in Icelandic.

• þögn = silence
• þögnin = the silence

So the sentence is talking about a specific silence, not silence in general.

Varð is the past tense of verða, which often means become.

So:

• verða = to become
• varð = became

In this sentence, Þögnin varð svo vandræðaleg... means The silence became so awkward...

Because vandræðaleg has to agree with Þögnin.

Þögn is:

• feminine
• singular
• nominative here

So the adjective also appears in the feminine singular nominative form:

• masculine: vandræðalegur
• feminine: vandræðaleg
• neuter: vandræðalegt

That is why Icelandic uses vandræðaleg here.

This is a very common Icelandic pattern meaning so ... that.

• svo vandræðaleg að... = so awkward that...

So the structure is:

• svo = so
• adjective/adverb
• að = that

In the sentence:

• Þögnin varð svo vandræðaleg að...
• The silence became so awkward that...

They do two different jobs.

1. ...svo vandræðaleg að enginn vissi...

   • here að means that

2. ...hann átti að segja

   • here að is the infinitive marker, like to in English

So:

• first að = that
• second að = to

This is very normal in Icelandic.

Because enginn is grammatically singular, even though its meaning refers to no person at all.

So Icelandic treats it like a singular subject:

• enginn vissi = nobody knew

That is why the verb is singular too:

• vissi = singular past tense

This is similar to English in sentences like nobody knows.

Because enginn is singular, and here it is in the masculine form, so the later pronoun matches it:

• enginn = masculine singular no one / nobody
• hann = he, matching that grammatical form

In this kind of sentence, hann does not necessarily mean an actual male person. It can be a generic singular pronoun referring back to enginn.

For a learner, the main point is: the pronoun agrees grammatically with enginn.

This is an important Icelandic construction:

• eiga að + infinitive

It often means:

• be supposed to
• should
• sometimes be meant to

Here:

• hann átti að segja = he was supposed to say
• in smoother English in this sentence: he knew what to say / what he should say

Since átti is past tense, the whole idea is also in the past: what he was supposed to say.

Because this is an embedded question, not a direct question.

Compare:

• direct question: Hvað átti hann að segja? = What was he supposed to say?
• embedded question: ...enginn vissi hvað hann átti að segja = ...nobody knew what he was supposed to say

In embedded questions, Icelandic normally uses:

• question word
• subject
• finite verb

So:

• hvað hann átti að segja

That word order is completely normal after a verb like vissi.

Hvað means what, and here it introduces an embedded question:

• enginn vissi hvað hann átti að segja
• nobody knew what he was supposed to say

So it does not function like a relative pronoun such as that which. It is simply the question word what inside a larger sentence.

Yes. The Icelandic meaning stays the same, but natural English can vary a bit. Possible translations include:

• The silence became so awkward that nobody knew what he was supposed to say.
• The silence became so awkward that no one knew what to say.
• The silence grew so awkward that nobody knew what he should say.

The most literal version keeps hann as he, but in natural English many speakers would simply say what to say.