Við athugum staðgreiðsluna því að upplýsingin er röng.

Because athugum is the correct present-tense form for við (we).

The verb is að athuga = to check / to examine.

Present tense:

• ég athuga = I check
• þú athugar = you check
• hann/hún/það athugar = he/she/it checks
• við athugum = we check
• þið athugið = you (plural) check
• þeir/þær/þau athuga = they check

So við athugum means we check or we are checking, depending on context.

Usually it can mean either, depending on context.

In Icelandic, the simple present often covers both:

• we check
• we are checking

So Við athugum ... can mean:

• We check ...
• We are checking ...

If you specifically want to emphasize an ongoing action, Icelandic can also use:

• Við erum að athuga ... = We are checking ...

But the simple present við athugum is completely normal.

Because it is the definite direct object of the verb.

The base noun is:

• staðgreiðsla = a withholding payment / the withholding tax system / direct withholding (depending on context)

Here it appears as:

• staðgreiðsluna

This form shows two things:

1. accusative case — because it is the object of athugum
2. definite article — the

So:

• staðgreiðsla = a withholding payment
• staðgreiðsluna = the withholding payment / the withholding tax

For many feminine nouns, the accusative singular definite ends in -una.

The dictionary form staðgreiðsla ends in -a, which is very often a feminine pattern in Icelandic.

A common feminine pattern is:

• nominative singular: -a
• accusative singular definite: -una

So:

• staðgreiðsla
• staðgreiðsluna

You still need to learn noun gender individually, but this ending is a strong clue.

Því að means because.

So:

• Við athugum staðgreiðsluna því að upplýsingin er röng.
• We check the withholding payment because the information is wrong.

This is a very common way to introduce a reason.

You may also see:

• af því að = because

In many situations, því að and af því að are very similar in meaning.

Yes, functionally it works very much like because.

Structure:

• main clause + því að
  • reason clause

So here:

• Við athugum staðgreiðsluna = main clause
• því að upplýsingin er röng = reason clause

Literally:

• We check the withholding payment because the information is wrong.

Because -in is the suffixed definite article, meaning the.

So:

• upplýsing = information / a piece of information
• upplýsingin = the information / the piece of information

Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

So compare:

• bók = book
• bókin = the book

Likewise:

• upplýsing
• upplýsingin

Because the adjective has to agree with the noun in gender, number, and case.

Here:

• upplýsingin is feminine singular
• so the adjective must also be feminine singular

That gives:

• röng = wrong (feminine singular)

Compare:

• rangur = masculine singular
• röng = feminine singular
• rangt = neuter singular

So:

• upplýsingin er röng = the information is wrong

This is just part of Icelandic adjective inflection.

The adjective rangur (wrong) changes form depending on the noun it describes.

Singular nominative forms:

• rangur — masculine
• röng — feminine
• rangt — neuter

Since upplýsingin is feminine, you need röng.

This kind of vowel change is common in Icelandic and is something learners simply get used to over time.

Because this is a normal subordinate clause introduced by því að.

The clause:

• upplýsingin er röng

has the ordinary pattern:

• subject + verb + adjective/complement

So:

• upplýsingin = subject
• er = is
• röng = wrong

That is completely normal Icelandic word order here.

It is nominative singular definite.

That is because it is the subject of:

• er röng = is wrong

In Icelandic, the subject of a clause is often in the nominative case.

So:

• upplýsingin = the information
• nominative singular feminine

Yes. That would also be natural, but it changes the nuance slightly.

• Við athugum staðgreiðsluna ... = We check / We are checking the withholding payment ...
• Við erum að athuga staðgreiðsluna ... = We are in the process of checking the withholding payment ...

The second version puts more emphasis on the ongoing action.

Yes, that is a very good question.

In Icelandic, upplýsingar (plural) is very common and often corresponds to English information in a general sense.

But upplýsing in the singular can mean:

• a piece of information
• an item of information

So upplýsingin er röng is understandable as:

• the information is wrong
• or more literally, the piece of information is wrong

Learners often notice this because English information is uncountable, while Icelandic often uses the plural upplýsingar in many everyday contexts.

More or less, yes:

• Við = we
• athugum = check / are checking
• staðgreiðsluna = the withholding payment / the withholding tax
• því að = because
• upplýsingin = the information
• er = is
• röng = wrong

So the sentence structure is fairly close to English, even though Icelandic marks grammar with endings much more than English does.