Endurskoðunin er á morgun.

In Icelandic, the definite article is usually attached to the end of the noun instead of being written as a separate word.

• endurskoðun = review / audit / revision
• endurskoðunin = the review / the audit / the revision

So the -in at the end is doing the job that the does in English.

The dictionary form is endurskoðun.

In this sentence, endurskoðunin is:

• singular
• definite
• nominative
• feminine

It is in the nominative because it is the subject of the sentence: The review is tomorrow.

A learner looking this word up in a dictionary should search for endurskoðun, not endurskoðunin.

Er is the present tense of vera (to be), but Icelandic often uses the present tense for scheduled or planned future events when a time expression makes the meaning clear.

So:

• Endurskoðunin er á morgun = The review is tomorrow

This works much like English:

• The meeting is tomorrow
• My exam is on Friday

You do not need a special future form here because á morgun already shows that the time is future.

Á morgun is a very common fixed expression meaning tomorrow.

It is best learned as a set phrase. Word-for-word, it is built with the preposition á plus a form of morgunn (morning), but in modern Icelandic the whole expression simply means tomorrow.

A very important contrast is:

• á morgun = tomorrow
• í morgun = this morning

English speakers often mix those up, so this is worth memorizing early.

Yes, that is correct.

Both of these are natural:

• Endurskoðunin er á morgun.
• Á morgun er endurskoðunin.

The difference is mainly emphasis:

• Endurskoðunin er á morgun focuses first on the review
• Á morgun er endurskoðunin focuses first on tomorrow

When you put á morgun first, the finite verb er still stays in second position. That is a very important Icelandic word-order pattern.

Yes, you can.

• Endurskoðunin er á morgun = very normal for schedules and arrangements
• Endurskoðunin verður á morgun = also correct, and can sound a bit more explicit as will be / will take place tomorrow

In many everyday contexts, er is the more natural, simple choice if the event is already arranged.

Usually, not as well in this context.

Icelandic has no separate word for a/an, so an indefinite noun is often just the bare noun:

• endurskoðun = a review / review

But in this sentence, endurskoðunin strongly suggests a specific, known review that both speaker and listener can identify. That is why the definite form is natural.

A bare Endurskoðun er á morgun would sound less natural unless the context made the meaning very clear, or unless it was being used in a special style such as a heading or note.

Because the sentence is built like this:

• Endurskoðunin = subject
• er = verb
• á morgun = time expression

So the structure is basically:

• The review
  • is
    • tomorrow

This is a very common Icelandic sentence pattern: noun + vera + time/place expression.