Hún efast um þetta svar.

• Hún = subject, she
• efast = verb, doubts / has doubts
• um = preposition, here required by the verb
• þetta svar = the thing she doubts, literally this answer

So the structure is:

Subject + verb + prepositional phrase

Icelandic often uses a preposition where English would just use a direct object.

Because að efast um e-ð is the normal Icelandic pattern for to doubt something.

So Icelandic expresses this idea as:

• efast um eitthvað = doubt something / have doubts about something

Even though English says doubt this answer, Icelandic says more literally doubt about this answer.

This is just something you learn with the verb: efast usually goes with um.

The -st ending is a very common Icelandic verb ending, often called the middle voice ending.

In some verbs it has a clear reflexive or passive-like meaning, but in many cases it is simply part of the normal dictionary form of the verb. Efast is one of those verbs you should learn as a whole unit.

So here, don’t try to translate -st separately. Just learn:

• að efast um = to doubt

Here it is present tense, agreeing with hún:

• Hún efast = She doubts

The infinitive is also að efast, so the form looks the same. That can be confusing at first.

In this sentence, you know it is present tense because it comes after the subject hún in a normal finite clause.

Because þetta is the correct form of the demonstrative þessi for a neuter singular noun in the nominative/accusative.

The noun svar is:

• neuter
• singular

So the demonstrative must match it:

• þetta svar = this answer

A learner often first meets the dictionary form þessi, but the word changes form depending on gender, number, and case.

It is in the accusative, because the preposition um normally takes the accusative.

So in:

• um þetta svar

both words are accusative.

However, svar is a neuter noun whose nominative and accusative singular look the same, so you do not see a visible change in the noun itself.

Yes. In Icelandic, demonstratives and adjectives must agree with the noun they modify in:

• gender
• number
• case

So þetta matches svar because svar is:

• neuter
• singular
• accusative here

That kind of agreement is much more visible in Icelandic than in English.

Because Icelandic usually does not use the suffixed definite article together with a demonstrative like þetta.

So you say:

• þetta svar = this answer

not something like:

• þetta svarið

except in special emphatic or stylistically marked contexts.

In normal usage, the demonstrative already makes the noun definite enough.

Yes, Icelandic word order is more flexible than English, but Hún efast um þetta svar is the most neutral and straightforward order.

Because Icelandic is a verb-second language, other orders are possible if you want to emphasize something, for example:

• Um þetta svar efast hún.

That would sound more marked, with focus on um þetta svar.

For a learner, the basic version is the best one to use first:

• Hún efast um þetta svar.

The two unusual letters are often what English speakers notice first:

• þ is like unvoiced th in thing
• ð is like voiced th in this (though in actual Icelandic pronunciation it can be softer or even disappear in some positions)

So:

• þetta sounds roughly like THET-ta with the first sound like thing, not like English z or t
• hún has a long u sound somewhat like oo in moon

A rough English-style approximation of the whole sentence is:

• Hoon EV-ast um THET-ta svar

But it is best to listen to native audio, since Icelandic vowels and stress do not match English exactly.

Yes, Icelandic does have related ways to express doubt, but efast um is one of the most common and natural patterns.

So for learners, the safest takeaway is:

• learn að efast um e-ð

as a set phrase meaning:

• to doubt something
• to have doubts about something

That will sound natural in many everyday situations.