Ég er í nýjum gallabuxum og hvítum stuttermabol í dag, af því að það er hlýtt úti.

In Icelandic, vera í literally means to be in, but it is also the normal way to say to be wearing clothes.

So:

• Ég er í nýjum gallabuxum = I am wearing new jeans
• Hann er í jakka = He is wearing a jacket

This is a very common Icelandic pattern. English usually uses wear, but Icelandic often uses be in for clothing.

Because both adjectives are agreeing with nouns after í, and here í takes the dative case.

So:

• nýjum gallabuxum = dative plural
• hvítum stuttermabol = dative singular masculine

Icelandic adjectives have to match the noun in:

• gender
• number
• case

That is why the adjective endings change instead of staying in one fixed form like in English.

Because gallabuxur means jeans, and like English jeans, it is normally treated as a plural word.

Its basic form is:

• gallabuxur = jeans

After í, it becomes dative plural:

• í gallabuxum = in / wearing jeans

So even if you are talking about one item of clothing, Icelandic still uses the plural word.

The dictionary form is stuttermabolur = T-shirt.

But after í, you need the dative singular, and many masculine nouns lose -ur in oblique cases.

So:

• stuttermabolur = nominative singular
• stuttermabol = dative singular

That is why the sentence has í hvítum stuttermabol.

It is the same word, but it is being used differently.

• í nýjum gallabuxum: í is used with clothing and takes the dative
• í dag: this is a fixed time expression meaning today

So you should really learn í dag as a whole phrase. It does not mean that the speaker is somehow inside a day in the same way they are in clothes. It is just the normal Icelandic expression for today.

Af því að means because.

It introduces the reason:

• Ég er í nýjum gallabuxum og hvítum stuttermabol í dag
  af því að það er hlýtt úti

= I’m wearing new jeans and a white T-shirt today because it is warm outside.

You may also see því að in Icelandic, but af því að is a very common and clear way to say because.

This is a very common weather-style pattern in Icelandic.

• það er hlýtt = it is warm

The það here works like English it in sentences such as:

• It is cold
• It is raining
• It is nice outside

It does not refer to a specific thing. It is just a grammatical subject.

Also, hlýtt is the neuter singular form of the adjective, which is the normal form used in this kind of impersonal statement.

Because adjective forms in Icelandic change depending on grammar.

The basic forms are roughly:

• hlýr = masculine
• hlý = feminine
• hlýtt = neuter

In það er hlýtt, the adjective is being used in an impersonal weather expression, and Icelandic uses the neuter singular form there.

So hlýtt is exactly the form you expect in a sentence like it is warm.

Because úti means outside / outdoors, describing location.

• úti = outside, outdoors
• út = out, outward, indicating movement

So:

• Það er hlýtt úti = It is warm outside
• Ég fer út = I am going out

A good shortcut is:

• úti = being outside
• út = going out

Yes, in everyday Icelandic you can often hear vera með used for wearing or having something on.

For example, people may say something like:

• Ég er með nýjar gallabuxur og hvítan stuttermabol

That is very natural in speech. But there is an important difference:

• vera í is the classic, straightforward way to say be wearing
• vera með is very common in conversation, but it behaves differently grammatically

For a learner, vera í is a very useful pattern to master first when talking about clothes.

Yes. Icelandic word order is flexible, and the time phrase can easily move to the front.

Both of these are natural:

• Ég er í nýjum gallabuxum og hvítum stuttermabol í dag
• Í dag er ég í nýjum gallabuxum og hvítum stuttermabol

When í dag comes first, the verb usually comes next:

• Í dag er ég...

That is a very common Icelandic word-order pattern.

These two letters are very important in Icelandic:

• þ is like the th in think
• ð is like the th in this, although in some positions it can sound very light

So:

• því starts with the think sound
• það starts with the same þ sound, and the ð is softer

For English speakers, the good news is that both sounds already exist in English. The challenge is mostly getting used to seeing them written as separate letters.