Í janúar er oft kaldast á morgnana.

In Icelandic, general statements like weather or temperature often use a dummy subject such as það, but when another element is placed first, that dummy það is often left out.

So this sentence can be understood as something like:

Það er oft kaldast á morgnana í janúar.

But when Í janúar is moved to the front, Icelandic normally just says:

Í janúar er oft kaldast á morgnana.

So English needs it, but Icelandic often does not.

Because Icelandic main clauses usually follow the verb-second rule.

That means:

• one element comes first
• the finite verb comes second

Here:

• Í janúar = first element
• er = second element

So the order is natural Icelandic word order. If you start the sentence with a time phrase, the verb still has to stay in second position.

Kaldast is the superlative form of kaldur (cold).

A simple way to think of the forms is:

• kaldur = cold
• kaldari = colder
• kaldastur = coldest

But in general or impersonal statements like this one, Icelandic often uses the neuter singular form:

• kalt = cold
• kaldara = colder
• kaldast = coldest

So kaldast here means coldest in the sense of it is coldest.

Because this is not describing a masculine noun like a coldest day or the coldest boy. It is part of an impersonal statement about the weather/temperature.

In those cases, Icelandic usually uses the neuter singular predicate form:

• Það er kalt = it is cold
• Það er kaldast = it is coldest

So kaldastur would not fit this sentence.

Oft means often, and its position here is normal Icelandic word order.

In a sentence like this, a common pattern is:

time phrase + verb + adverb + complement

So:

• Í janúar
• er
• oft
• kaldast
• á morgnana

English and Icelandic do not always place adverbs in exactly the same spot, so this is a good pattern to notice and remember.

It is a time expression meaning in January or during January.

With months, Icelandic normally uses í:

• í janúar
• í febrúar
• í mars

So this is a very common pattern to memorize as a chunk: í + month.

Because á morgnana means in the mornings as a habitual or general time.

The plural gives the sense of repeated occasions:

• not one specific morning
• but mornings in general

That is why it matches English in the mornings very well.

A singular form would usually suggest a different meaning, and á morgun actually means tomorrow, so that is a completely different expression.

It is part of the definite article attached to the noun.

The base noun is morgunn (morning).
In this expression, morgnana is the accusative plural definite form, literally something like the mornings.

So:

• morgnar = mornings
• morgnana = the mornings

In the phrase á morgnana, this form is used idiomatically to mean in the mornings.

Because á morgnana is the standard Icelandic expression for in the mornings.

Prepositions in time expressions are often idiomatic, just like in English:

• in the morning
• at night
• on Monday

Icelandic has its own patterns, and á morgnana is one of them. It is best learned as a fixed phrase.

Not necessarily. It usually means in the mornings generally, mornings are often the coldest time, or as a usual pattern.

So it expresses a general tendency, not a strict claim about every single morning without exception.

Yes. A sentence like:

Það er oft kaldast á morgnana í janúar.

would be very close in meaning.

The version with Í janúar first simply gives January a bit more prominence. That fronting is very natural in Icelandic and works with the verb-second rule.