Ef hún fær ekki svar í dag, bankar hún aftur á morgun.

In Icelandic, it’s standard to put a comma between a subordinate clause and the main clause when the subordinate clause comes first.
So Ef hún fær ekki svar í dag, (subordinate clause) is followed by a comma before bankar hún aftur á morgun (main clause).

Ef is the common word for if and introduces a conditional subordinate clause.
The pattern here is:

• Ef
  • clause, comma, + main clause

So the first part sets the condition under which the second part happens.

Icelandic follows a V2 (verb-second) rule in main clauses: the finite verb usually comes second.
When something other than the subject comes first (here, the whole Ef-clause), the main clause typically begins with the verb:

• (Condition first) → bankar (verb) hún (subject)

So bankar hún is the normal word order after a fronted clause.

Ekki (not) usually comes right after the finite verb (or after the subject+finite verb combination), and before objects and many adverbs.
Here:

• fær ekki svar = gets not an answer

That placement is very typical in Icelandic.

Icelandic commonly uses the present tense to talk about future time when a time expression makes it clear (like í dag, á morgun).
So bankar hún ... á morgun is understood as she will knock ... tomorrow without needing a special future tense.

Svar is the object of fá (to get/receive), and fá normally takes an accusative object.
In this particular word (svar, neuter singular), the nominative and accusative forms look the same, so you don’t “see” the case from the ending—but the verb’s usual case pattern tells you it’s accusative.

In Icelandic you normally include the subject in each clause:

• Ef hún ... , bankar hún ...

You generally can’t omit the subject the way some languages do, because Icelandic isn’t a pro-drop language in the everyday sense. Leaving out hún would sound incomplete or would require a different structure.

Aftur means again here. Its placement is flexible, but common spots are:

• bankar hún aftur á morgun (very natural)
• bankar hún á morgun aftur (also possible; can feel slightly more emphatic on the time phrase depending on context)

In this sentence, placing aftur before á morgun is a neutral, typical choice.

This is idiomatic Icelandic usage:

• í dag = today (literally “in day”)
• á morgun = tomorrow (literally “on morning/tomorrow”)

You mainly learn these as fixed expressions. (Similarly: í gær = yesterday, í kvöld = tonight.)

The sentence uses 3rd person singular present:

• að fá → hún fær (she gets)
• að banka → hún bankar (she knocks)

Helpful present-tense pattern reminders:

• fá is irregular: ég fæ, þú færð, hann/hún/það fær
• banka is regular: ég banka, þú bankar, hann/hún/það bankar