Það var óvenjulegur morgunn, en eftir hádegi varð allt aftur venjulegt.

Because it agrees with morgunn.

Morgunn is a masculine singular noun here, in the nominative case, so the adjective also has to be masculine singular nominative:

• óvenjulegur morgunn = an unusual morning

In Icelandic, adjectives change form much more than in English. A native English speaker often expects one adjective form everywhere, but Icelandic adjectives must match the noun in gender, number, and case.

Because venjulegt agrees with allt, not with morgunn.

Here:

• allt = everything
• grammatically, allt is neuter singular

So the adjective must also be neuter singular:

• allt varð aftur venjulegt = everything became normal again

This is a very common Icelandic pattern: a predicate adjective still agrees with the subject.

They come from two different verbs:

• var = past tense of vera = was
• varð = past tense of verða = became / turned

So the sentence contrasts:

• Það var óvenjulegur morgunn = It was an unusual morning
• allt varð aftur venjulegt = everything became normal again

A useful shortcut is:

• vera = being in a state
• verða = changing into a state

Here það works a lot like English it in sentences such as It was a strange morning.

It does not necessarily point to a specific object. It is an introductory pronoun that helps set up the sentence naturally. Icelandic often uses það var... in a similar way to English it was....

So:

• Það var óvenjulegur morgunn
  means something like
• It was an unusual morning

even though the real content is in óvenjulegur morgunn.

Because Icelandic is a verb-second language in main clauses.

That means the finite verb usually comes in the second position, even if something other than the subject comes first.

So in the second clause:

• eftir hádegi = first element
• varð = verb in second position
• allt = subject after the verb

This is why Icelandic says:

• eftir hádegi varð allt aftur venjulegt

not:

• eftir hádegi allt varð...

This is one of the most important word-order patterns for English speakers to learn.

Here aftur means again or back.

So:

• allt varð aftur venjulegt

means:

• everything became normal again
• or more naturally, everything went back to normal

In other contexts, aftur can also mean back in a physical sense, but here it refers to a return to an earlier state.

Because Icelandic does not have an indefinite article like English a/an.

So a bare noun phrase can mean:

• morgunn = morning / a morning
• óvenjulegur morgunn = an unusual morning

Icelandic does have a definite article, but it is usually attached to the noun as an ending rather than written as a separate word.

So English an unusual morning is simply:

• óvenjulegur morgunn

It means after noon or in the afternoon / after midday, depending on context.

• eftir = after
• hádegi = noon / midday

So the phrase sets the time for the second part of the sentence:

• en eftir hádegi varð allt aftur venjulegt
  = but after noon everything became normal again

This is a very natural time expression in Icelandic.

In this time expression, hádegi is used after the preposition eftir. For many learners, the important point is that the form stays hádegi, so there is no visible ending change here.

In other words, even if you are thinking about case, this is not a form where the noun obviously looks different. So the phrase is best learned as a chunk:

• eftir hádegi = after noon

That is often the most practical approach at this stage.

Here allt means everything.

It comes from allur = all, but in this form it is being used on its own, not before a noun:

• allt = everything

Because allt is grammatically neuter singular, it causes the adjective after it to be neuter singular too:

• allt ... venjulegt

So this is not just a vocabulary point; it also affects the grammar of the sentence.

The prefix ó- often gives a negative meaning, similar to English un-.

So:

• venjulegur = normal / usual / ordinary
• óvenjulegur = unusual / out of the ordinary

This is a very useful pattern in Icelandic. Once you know ó-, you can recognize many opposites more easily.

In English, everything was normal again may sound natural, but the Icelandic specifically uses varð, which means became.

So the Icelandic emphasizes a change back to normal:

• varð aftur venjulegt = became normal again

If Icelandic had used var, the meaning would focus more on simply being in that state, not on the change into it.

So became normal again is the grammatically closest match, even if English sometimes prefers a smoother translation like was back to normal.