Mjólkin er á tilboði í dag.

Because -in is the definite article attached to the end of the noun.

• mjólk = milk
• mjólkin = the milk

In Icelandic, the is usually not a separate word like in English. Instead, it is added to the noun itself.

Here, mjólkin is the nominative singular definite form.

Mjólk is a feminine noun, and yes, that matters.

In Icelandic, nouns have grammatical gender:

• masculine
• feminine
• neuter

The gender affects:

• the form of the definite article
• adjectives
• pronouns
• some inflection patterns

Because mjólk is feminine, its definite form is mjólkin.

Er means is.

It is the 3rd person singular present tense of vera (to be).

So:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is

In this sentence:

• Mjólkin er ... = The milk is ...

Á tilboði is a very common expression meaning:

• on sale
• on special offer
• being sold at a discount

Word-for-word, it is not something you should translate too literally. It is best learned as a set phrase.

• tilboð = offer
• á tilboði = on offer / on sale

So the sentence means that the milk is being sold at a reduced price today.

Because the noun changes form after the preposition á in this expression.

The basic form is:

• tilboð = offer

But in á tilboði, Icelandic uses the dative singular:

• tilboði

This is very common in Icelandic: prepositions often require a certain case.

Here, á is part of the fixed expression vera á tilboði.

Í dag means today.

It is a fixed time expression:

• í dag = today
• í gær = yesterday
• á morgun = tomorrow

Even though í often means in, in this phrase you should just learn í dag as the normal Icelandic way to say today.

This is the normal straightforward word order:

• Mjólkin = subject
• er = verb
• á tilboði = complement/expression
• í dag = time expression

So it follows a pattern like: Subject + Verb + More information

A very natural alternative is:

• Í dag er mjólkin á tilboði.

That puts extra emphasis on today.

Icelandic often follows verb-second word order, so if you move í dag to the front, the verb er still comes next:

• Í dag er mjólkin á tilboði.

Grammatically, mjólk without the article is possible in some contexts, but it changes the feel.

• Mjólkin er á tilboði í dag = the milk is on sale today
• Mjólk er á tilboði í dag = more like milk is on sale today

The version with mjólkin sounds more like a specific product or item being referred to, which is very natural in a shop or advertisement.

So the sentence as given is not wrong or unusual at all—it is a very normal way to say it.

A rough English-friendly approximation is:

MYOHL-kin

A few pronunciation notes:

• mj at the start is pronounced together, something like my
• ó is a long vowel, roughly like oh
• stress in Icelandic is usually on the first syllable

So:

• Mjólkin ≈ MYOHL-kin

The exact Icelandic pronunciation is subtler than English spelling can show, but that approximation is a good start.

In this sentence, á tilboði means on sale in the sense of discounted or on special offer, not just available to buy.

English for sale usually means available to be bought.

So:

• á tilboði = on special offer / discounted
• not usually just for sale

That distinction is important.

Mjólkin is in the nominative singular.

It is the subject of the sentence:

• Mjólkin er ... = The milk is ...

Subjects are normally in the nominative case in Icelandic.

So the structure is:

• mjólkin = nominative subject
• er = verb
• á tilboði = prepositional phrase/expression
• í dag = time expression

Yes, you can absolutely say it in everyday speech.

It works well in contexts like:

• shopping
• talking about prices
• supermarket offers
• discussing what to buy today

For example:

• Eigum við að kaupa mjólk? Mjólkin er á tilboði í dag.
  • Should we buy milk? The milk is on sale today.

So it is natural both in advertisements and in normal conversation.