Ég athuga inneignina mína í netbankanum áður en ég borga reikninginn.

Because it is the direct object of athuga in this sentence.

• Base noun: inneign = balance, credit, available funds
• Gender: feminine
• Number: singular
• Case here: accusative, because athuga takes a direct object
• Definite: yes, so it means the balance

So:

• inneign = balance
• inneignina = the balance (feminine singular accusative definite)

That is why you see the ending -ina.

In Icelandic, possessives are very often placed after a definite noun:

• inneignina mína = my balance
• literally: the-balance my

The form mína has to agree with inneignina in:

• gender: feminine
• number: singular
• case: accusative

So it is not just the basic dictionary form minn/mín/mitt. It changes to match the noun.

Here the matching form is:

• mína = feminine singular accusative

A learner-friendly way to think about it is:

• inneignin mín = my balance, if the noun is nominative
• inneignina mína = my balance, when it is accusative in this sentence

Because í can take either accusative or dative, depending on meaning.

A very common rule is:

• location / being in something → dative
• movement into something → accusative

Here the meaning is location:

• í netbankanum = in the online bank / in the online banking system

So Icelandic uses the dative.

That is why you get netbankanum, not an accusative form.

It comes from the noun netbanki, which means online bank or more naturally online banking system / internet bank portal.

Breakdown:

• netbanki = online bank
• dative singular stem form: netbanka
• definite article added: -num

So:

• netbankanum = in the online bank / in the online banking portal

This is also a good reminder that Icelandic usually attaches the to the end of the noun instead of using a separate word.

Áður en means before when it introduces a clause.

So:

• áður en ég borga reikninginn = before I pay the bill

It is a very common way to link two actions in time:

1. I check the balance
2. then I pay the bill

So the first action happens áður en the second one.

Because after áður en, you have a subordinate clause, and subordinate clauses do not use the normal main-clause verb-second pattern in the same way.

So the usual order is:

• áður en ég borga reikninginn
• before I pay the bill

with:

• ég = subject
• borga = verb

This is very natural Icelandic subordinate-clause word order.

Because it is the direct object of borga.

The base noun is:

• reikningur = bill, invoice

In this sentence it is:

• singular
• accusative
• definite

So:

• reikningur = a bill
• reikninginn = the bill

The ending -inn is the masculine singular accusative definite ending here.

In sentences like this, yes: the thing being paid is commonly in the accusative.

Examples:

• borga reikninginn = pay the bill
• borga skattinn = pay the tax

But borga can appear with other patterns too, especially when a person is involved as the recipient.

For this sentence, the important pattern is:

• borga + thing paid in the accusative

So reikninginn is exactly what you should expect.

Athuga means to check, to examine, or to look at carefully.

Here it is the present tense, first person singular:

• ég athuga = I check

This verb is convenient for learners because the ég form looks the same as the infinitive:

• að athuga = to check
• ég athuga = I check

In this sentence, it means something like checking your balance before making a payment.

Because Icelandic usually puts the definite article onto the end of the noun.

In this sentence you can see it several times:

• inneignina = the balance
• netbankanum = the online bank / the online banking portal
• reikninginn = the bill

So instead of a separate word like English the, Icelandic often uses an ending.

This is one of the most important differences between English and Icelandic noun phrases.

Yes. Icelandic allows different word orders for emphasis, especially in main clauses.

For example, you could say:

• Áður en ég borga reikninginn, athuga ég inneignina mína í netbankanum.

That still means the same thing.

But notice an important Icelandic rule: in a main clause, the finite verb usually stays in second position. So when Áður en ég borga reikninginn is moved to the front, you get:

• athuga ég
• not ég athuga

That verb-second pattern is very important in Icelandic.