Ég ætla að hringja í hana á morgun.

Ætla að is a very common way to express an intention or a planned future action.

So:

• Ég ætla að hringja = I’m going to call
• literally, it is something like I intend to call

In everyday Icelandic, this is often more natural than trying to make a separate future tense.

Here, að is the infinitive marker, like English to in to call.

• ætla að hringja = intend to call / am going to call

The verb hringja is in the infinitive form, and after ætla you normally use að + infinitive.

Compare:

• Ég ætla að fara = I’m going to go
• Ég ætla að borða = I’m going to eat
• Ég ætla að hringja = I’m going to call

Because in Icelandic, the verb hringja usually takes the preposition í when you mean call someone on the phone.

So the pattern is:

• hringja í einhvern = to call someone

Examples:

• hringja í mig = call me
• hringja í hann = call him
• hringja í hana = call her

This is just something you learn as part of the verb’s pattern, much like English learners have to learn which prepositions go with which verbs.

Because hana is the object form of hún.

• hún = she (subject form)
• hana = her (object form)

In this sentence, her is not doing the action; she is receiving it, so Icelandic uses the object form:

• Ég ætla að hringja í hana = I’m going to call her

You can think of it like:

• hún hringir = she calls
• ég hringi í hana = I call her

Á morgun is the normal expression for tomorrow.

Even though á often means on, you should learn á morgun as a fixed time expression meaning tomorrow.

A useful contrast is:

• í morgun = this morning
• á morgun = tomorrow

So:

• Ég hringdi í hana í morgun = I called her this morning
• Ég ætla að hringja í hana á morgun = I’m going to call her tomorrow

Yes, á morgun can move, and Icelandic word order is somewhat flexible.

This sentence is the neutral, straightforward order:

• Ég ætla að hringja í hana á morgun.

But you can also say:

• Á morgun ætla ég að hringja í hana.

That version emphasizes tomorrow a bit more.

One important thing to notice: if something else comes first, Icelandic usually puts the finite verb second. So:

• Ég ætla ...
• Á morgun ætla ég ...

That verb-second pattern is very important in Icelandic.

Yes, it is very natural.

Icelandic often uses present-tense forms plus context, or expressions like ætla að, rather than a dedicated future tense in the way English does.

So this sentence sounds very normal for:

• a plan
• an intention
• something the speaker expects to do

Another possible future expression is munu, but that often sounds more formal, more definite, or more like a prediction:

• Ég mun hringja í hana á morgun. = I will call her tomorrow.

In everyday speech, Ég ætla að hringja í hana á morgun is extremely common.

Ætla is the present tense, first person singular:

• ég ætla = I intend / I’m going to

A very small present-tense set is:

• ég ætla = I intend
• þú ætlar = you intend
• hann/hún ætlar = he/she intends

So the sentence begins with the finite verb phrase:

• Ég ætla ... = I’m going to ...

Hringja is the infinitive form, meaning to call.

After ætla að, you use the infinitive:

• ætla að hringja = going to call

You are not seeing a tense-marked form here. The tense is carried by ætla.

So:

• ætla = the conjugated verb
• að hringja = infinitive phrase

In modern everyday Icelandic, hringja í einhvern usually means to phone someone / call someone.

The verb is connected historically with ringing, but in normal usage, if you say:

• Ég hringi í hana

people will usually understand:

• I’m calling her on the phone

You keep hringja í and change the pronoun:

• Ég ætla að hringja í hann á morgun. = I’m going to call him tomorrow.
• Ég ætla að hringja í þau á morgun. = I’m going to call them tomorrow.
• Ég ætla að hringja í mig would not normally make sense for I’m going to call myself, but grammatically mig is the form after í.

Some useful object forms after í here are:

• mig = me
• þig = you
• hann = him
• hana = her
• okkur = us
• ykkur = you all
• þau / þá / þær = them, depending on gender and group type

A rough learner-friendly guide might be:

• Ég ≈ yegg (with a palatal y-like start)
• ætla ≈ EYEHT-la or EYE-tla depending on speech
• að ≈ often a short ath / adh
• hringja ≈ roughly HRING-ya
• í = long ee
• hana ≈ HA-na
• á morgun ≈ ow MOR-gun

A few pronunciation notes:

• hr is voiceless at the start, which sounds unusual to English speakers.
• g in morgun is not always pronounced like hard English g.
• Unstressed words like að may sound reduced in normal speech.

If you want to sound natural, it helps to learn the whole phrase as one chunk:

• Ég ætla að hringja í hana á morgun.

Not usually for this meaning.

Ég er að fara að + infinitive means something like I’m about to ... or I’m just going to ... very soon.

So:

• Ég er að fara að hringja í hana = I’m about to call her

That clashes a bit with á morgun because about to suggests something immediate, while tomorrow is later.

For a plan for tomorrow, Ég ætla að hringja í hana á morgun is the better choice.

Yes—three very useful chunks:

• ég ætla að ... = I’m going to ...
• hringja í einhvern = call someone
• á morgun = tomorrow

If you memorize those as complete pieces, you can build many new sentences easily:

• Ég ætla að fara á morgun. = I’m going tomorrow.
• Ég ætla að tala við hana á morgun. = I’m going to talk to her tomorrow.
• Ég ætla að hringja í hann á morgun. = I’m going to call him tomorrow.