Veðrið er ekki eins kalt og í gær.

Veðrið means the weather.

The base noun is veður = weather.
The ending -ið is the definite article attached to the noun, so:

• veður = weather
• veðrið = the weather

Unlike English, Icelandic usually adds the as an ending instead of using a separate word.

In this sentence, veðrið is the subject: the weather.

Because veðrið is a neuter singular noun, and the adjective has to agree with it.

The adjective kaldur = cold has different forms:

• kaldur = masculine
• köld = feminine
• kalt = neuter

Since veður is neuter, you say:

• Veðrið er kalt = The weather is cold

This is very common in Icelandic: after vera (to be), the adjective still agrees with the noun.

That is the normal word order in a simple Icelandic main clause.

Here the structure is:

• Veðrið = subject
• er = verb
• ekki = not

So:

• Veðrið er ekki ...

In Icelandic, the finite verb typically comes early in the sentence, and ekki usually comes right after it.

Compare:

• Veðrið er kalt = The weather is cold
• Veðrið er ekki kalt = The weather is not cold

Eins ... og is the pattern for as ... as.

So:

• eins kalt og í gær = as cold as yesterday

With ekki, it becomes:

• ekki eins kalt og í gær = not as cold as yesterday

This is the standard way to make an equality comparison in Icelandic.

A few examples:

• hann er eins hár og ég = he is as tall as I am
• hún er ekki eins fljót og systir sín = she is not as fast as her sister

Yes, og usually means and, but in the fixed comparison pattern eins ... og, it works like English as in as ... as.

So here, og is not being used in the ordinary and sense.

This is worth remembering as a set phrase:

• eins ... og = as ... as

By contrast, when Icelandic uses a true comparative form such as colder, it usually uses en:

• kaldara en í gær = colder than yesterday

So:

• eins kalt og = as cold as
• kaldara en = colder than

Í gær means yesterday.

Even though í often means in, you should treat í gær as a fixed time expression. It does not translate word-for-word in a natural English way.

So just learn it as a chunk:

• í gær = yesterday

Other useful time expressions include:

• í dag = today
• á morgun = tomorrow

Yes, you can think of something as being understood.

A fuller version would be something like:

• Veðrið er ekki eins kalt og það var í gær.
• The weather is not as cold as it was yesterday.

In everyday Icelandic, just saying og í gær is completely natural, because the missing idea is obvious from context.

English does this too:

• It’s not as cold as yesterday instead of
• It’s not as cold as it was yesterday

Yes. That is also very natural.

Both of these work:

• Veðrið er ekki eins kalt og í gær.
• Það er ekki eins kalt og í gær.

The version with það er is often very common for weather and general conditions:

• Það er kalt = It is cold
• Það er rigning = It is raining / There is rain

The version with veðrið is a bit more explicit because it directly mentions the weather.

Not exactly, though the idea is close.

Veðrið er ekki eins kalt og í gær literally means:

• The weather is not as cold as yesterday

That strongly suggests it is warmer than yesterday, but the sentence is framed as a negative comparison.

If you want to say it directly with a comparative adjective, you would say:

• Veðrið er hlýrra en í gær. = The weather is warmer than yesterday.

So the difference is:

• ekki eins kalt og ... = not as cold as ...
• hlýrra en ... = warmer than ...

It is in the nominative, because it is the subject of the sentence.

The sentence is basically:

• Veðrið = the subject
• er = is
• ekki eins kalt og í gær = the rest of the predicate

So this is the normal subject form of the noun.

The main dictionary forms are:

• veður = weather
• vera = to be
• ekki = not
• kaldur = cold
• eins = as, equally
• og = and / as in comparisons
• gær = yesterday, in the expression í gær

This is useful because the sentence uses inflected forms:

• veðrið comes from veður
• er comes from vera
• kalt comes from kaldur