Mér mistókst að skrá mig inn í netbankann í morgun.

Because mistakast is often used in a pattern where the person is in the dative, not the nominative.

So Icelandic says:

• Mér mistókst ... = literally something like To me, it failed
• not Ég mistókst ...

This is a very common kind of thing in Icelandic: some verbs do not use a normal nominative subject the way English does.

In this sentence, mér means to me / for me, but in natural English we translate it simply as I:

• Mér mistókst að skrá mig inn ... = I failed to log in ...

The dictionary form is mistakast.

mistókst is the past tense form, and in this sentence it means failed or did not succeed.

A very useful pair is:

• mér tókst að ... = I managed to ... / I succeeded in ...
• mér mistókst að ... = I failed to ...

So mistakast is closely related to takast, but with the idea of failure rather than success.

Because að + infinitive is how Icelandic expresses to do something.

Here:

• að skrá mig inn = to log myself in / to log in

After mistakast, Icelandic very often uses this pattern:

• e-m mistakast að gera eitthvað = someone fails to do something

So the structure is:

• Mér mistókst
• að skrá mig inn í netbankann

In other words: I failed to log in to the online bank.

Because the Icelandic expression is skrá sig inn, literally register oneself in, and this is the normal way to say log in / sign in.

So the pronoun is part of the expression:

• skrá mig inn = log myself in
• skrá þig inn = log yourself in
• skrá sig inn = log himself/herself/themselves in

English usually does not say log myself in unless it is being very literal, but Icelandic commonly uses this reflexive-style structure.

Because sig is only the third-person reflexive pronoun.

So:

• mig = me
• þig = you
• sig = himself / herself / itself / themselves

In this sentence, the person trying to log in is I, so Icelandic uses mig:

• að skrá mig inn

If the sentence were about he/she, then it would be:

• Honum mistókst að skrá sig inn.

Inn is a particle meaning in / inside, and it works together with the verb phrase skrá sig inn.

Compare it loosely to English log in:

• the in is part of the verbal idea

Then í netbankann tells you where the login is directed:

• into the online bank

So inn is not random extra material. It is a normal part of the expression.

Because í can take either:

• accusative for motion/direction
• dative for location

Here Icelandic treats logging in as movement into a system, so it uses the accusative:

• inn í netbankann = into the online bank

If it were just a static location, you would expect dative:

• í netbankanum = in the online bank

So the accusative here matches the idea of entering/accessing the system.

Netbankann is the accusative singular definite form of netbanki.

Breakdown:

• netbanki = online bank / online banking portal
• netbankann = the online bank (accusative singular)

The ending -ann shows the definite form here.

This is required because:

• the noun is definite: the online bank
• the preposition phrase is directional: í netbankann

Because they do different jobs:

• í netbankann = a prepositional phrase showing direction/destination
• í morgun = a time expression meaning this morning

So even though both use í, they are not the same type of phrase in terms of meaning.

You can think of them like this:

• inn í netbankann = where?
• í morgun = when?

In this sentence, í morgun means this morning.

It is a very common fixed expression.

Examples:

• Ég fór í sund í morgun. = I went swimming this morning.
• Hann hringdi í morgun. = He called this morning.

Be careful not to confuse it with:

• á morgun = tomorrow

That is a very common learner mix-up.

Yes, you could, and it is very natural.

The difference is mainly nuance:

• Mér mistókst að skrá mig inn ... = I tried, but failed
• Ég gat ekki skráð mig inn ... = I couldn’t log in

So mistókst often suggests an attempt that did not succeed, while gat ekki is a more general could not.

In many real situations, both are possible, but mistókst can sound slightly more like I made an attempt and it didn’t work out.

The sentence follows a very normal Icelandic pattern, with the finite verb in second position:

• Mér | mistókst | að skrá mig inn í netbankann í morgun

You can move other elements for emphasis, but the finite verb still tends to stay in second position.

For example:

• Í morgun mistókst mér að skrá mig inn í netbankann.

That is also correct and natural, with more emphasis on this morning.

What you generally cannot do is replace mér with ég here, because the verb pattern itself requires the dative pronoun.

The dative pronoun changes, and the object pronoun in skrá ... inn changes too.

Examples:

• Þér mistókst að skrá þig inn. = You failed to log in.
• Honum mistókst að skrá sig inn. = He failed to log in.
• Henni mistókst að skrá sig inn. = She failed to log in.
• Okkur mistókst að skrá okkur inn. = We failed to log in.
• Ykkur mistókst að skrá ykkur inn. = You all failed to log in.
• Þeim mistókst að skrá sig inn. = They failed to log in.

Notice:

• the person after mistókst is in the dative
• the pronoun after skrá matches the person doing the logging in