Hún athugar brottförina aftur í símanum af því að veðrið er slæmt.

Hún is the nominative form, which is the form used for the subject of the sentence.

Here, hún is the person doing the action:

• Hún athugar ... = She checks ...

Other cases of hún appear when it has other roles:

• hana = her
• henni = to her / for her
• hennar = her / hers

So Hún is used because she is the subject.

Athugar is the present tense of the verb að athuga, meaning to check, to examine, or to look into.

With many Icelandic verbs, the 3rd person singular present ends in -ar:

• ég athuga = I check
• þú athugar = you check
• hún/hann/það athugar = she/he/it checks

In this sentence, the subject is hún, so the verb is correctly athugar.

Brottförina means the departure. The ending -ina shows that the noun is:

• definite = the departure, not just a departure
• in the accusative singular

That accusative form is used because brottförina is the direct object of athugar:

• athuga eitthvað = to check something

So:

• brottför = departure
• brottförin = the departure
• brottförina = the departure, as a direct object

Yes, it matters. Brottför is a feminine noun.

That affects:

• the form of the definite article
• the case endings

Here the form brottförina shows:

• feminine
• singular
• definite
• accusative

Icelandic nouns change form much more than English nouns do, so learning the gender of each noun is very important.

Aftur means again, and its placement here is natural Icelandic word order:

• Hún athugar brottförina aftur ...

This is similar to English:

• She checks the departure again ...

You will often see adverbs like aftur after the object or near the verb, depending on emphasis and sentence rhythm. In this sentence, it simply means she is checking it one more time.

The phrase í símanum literally means in the phone, but in natural English it is often best understood as on the phone or in her phone depending on context.

Grammatically:

• í = in
• símanum = the phone in the dative singular

After í, Icelandic uses:

• accusative for motion into something
• dative for location in something

Here there is no movement, just location, so it is í símanum.

Examples:

• í símann = into the phone
• í símanum = in the phone / on the phone

Because í takes different cases depending on meaning.

Here the phrase describes location:

• she is checking it in/on the phone

When í means location, it takes the dative:

• í símanum

If there were motion into something, it would take the accusative:

• setja það í símann = put it into the phone

So símanum is used because this is a static location, not movement.

Af því að is a very common way to say because.

It is made up of:

• af = of / from
• því = that / it, in a case form
• að = that / to

You do not need to translate it word by word in normal use. As a full expression, it simply means because.

You may also see:

• því að = because

Both are common, and af því að often feels especially natural in everyday speech.

Veðrið means the weather. The ending -ið is the definite article attached to the noun.

So:

• veður = weather
• veðrið = the weather

In this sentence, Icelandic uses the definite form:

• af því að veðrið er slæmt = because the weather is bad

This is very common in Icelandic, even where English might simply say because weather is bad in a very general sense. Icelandic normally wants the definite noun here.

Because it agrees with veðrið, which is a neuter singular noun.

In Icelandic, adjectives must agree with the noun they describe in:

• gender
• number
• case

Since veðrið is neuter singular, the adjective is also neuter singular:

• slæmt

Compare:

• slæmur = masculine singular
• slæm = feminine singular
• slæmt = neuter singular

So:

• veðrið er slæmt = the weather is bad

Er is the present tense of að vera = to be.

So:

• veðrið er slæmt = the weather is bad

This works just like English, where to be links the subject and the description:

• weather + is + bad

The adjective slæmt is a predicate adjective, meaning it describes the subject after er.

It is fairly close to English word order:

• Hún athugar brottförina aftur í símanum af því að veðrið er slæmt.
• She checks the departure again on the phone because the weather is bad.

So the overall structure is familiar:

1. subject
2. verb
3. object
4. adverb / prepositional phrase
5. reason clause

What is more distinctly Icelandic is not the basic order here, but the case marking and noun/adjective endings:

• brottförina
• símanum
• veðrið
• slæmt

Yes. Að athuga is a very useful everyday verb and can mean things like:

• check
• look at
• look up
• inspect
• make sure about
• see whether

So in this sentence, it could suggest that she is:

• checking the departure details
• checking whether it has changed
• looking it up again on her phone

The exact English translation depends on context, but check is a very natural core meaning.

Brottför means departure in a fairly general sense, but it is very commonly used in travel contexts, such as:

• flights
• buses
• ferries
• other scheduled departures

So in context, it could easily refer to:

• the departure time
• the departure listing
• the scheduled departure of a flight or other transport

If the larger context is air travel, learners will often understand brottförin as the departure or the outbound departure information.

It would be:

• Ég athuga brottförina aftur í símanum af því að veðrið er slæmt.

The main change is the subject:

• Hún = she
• Ég = I

The verb form athuga also changes:

• hún athugar = she checks
• ég athuga = I check

Everything else can stay the same.