Margir eru mjög þreyttir á mánudögum.

In Icelandic, margir can stand on its own and mean many people when the noun is understood from context.

So Margir eru mjög þreyttir... literally means something like Many are very tired..., but in natural English we usually say Many people are very tired...

This is common in Icelandic. An adjective can function almost like a pronoun when the noun is obvious or general.

Margir is the masculine nominative plural form of margur.

It is used here because the sentence refers to an unspecified group of people, and Icelandic often uses the masculine plural for mixed groups or for people in general.

Compare:

• margir = many (masculine / mixed people)
• margar = many (feminine)
• mörg = many (neuter)

So if you were specifically talking about many women, you might say margar instead.

Because the subject is plural.

The verb vera (to be) changes according to number:

• er = is
• eru = are

Since margir means many people, the verb has to be plural too:

• Margir eru... = Many are...

Þreyttur is the basic masculine singular form, the form you usually see in dictionaries.

In the sentence, the adjective has to agree with the subject margir, so it becomes þreyttir, which is the masculine nominative plural form.

This is because Icelandic adjectives agree with the noun or pronoun they describe in:

• gender
• number
• case

Here:

• margir = masculine, plural, nominative
• therefore þreyttir must also be masculine, plural, nominative

It means on Mondays.

The preposition á often means on, and with days of the week it is used to talk about time:

• á mánudag = on Monday
• á mánudögum = on Mondays

So in this sentence, á mánudögum tells you when many people are very tired.

It is plural because the sentence is talking about Mondays in general, as a repeated or habitual time:

• á mánudögum = on Mondays / on Mondays generally

The ending -um is the dative plural ending. After á in this kind of time expression, Icelandic uses the dative.

So:

• mánudagur = Monday
• mánudögum = Mondays / to Mondays / on Mondays, depending on context and preposition

In this sentence, it is specifically dative plural after á.

Usually it means on Mondays in general or on Mondays as a regular pattern.

It does not necessarily mean literally every single Monday without exception, but it does describe a general habit or tendency.

So the sentence gives a broad statement: people are often very tired when Monday comes around.

Mjög means very, and it is an adverb.

It comes before the adjective it modifies:

• mjög þreyttir = very tired

That placement is normal in Icelandic, just as in English:

• very tired
• mjög þreyttir

If you remove it, the sentence still works:

• Margir eru þreyttir á mánudögum. = Many are tired on Mondays.

Adding mjög just strengthens the meaning.

The given order is completely natural:

• Margir eru mjög þreyttir á mánudögum.

But Icelandic can move elements around for emphasis. For example:

• Á mánudögum eru margir mjög þreyttir.

This still means the same thing, but now on Mondays is placed first for emphasis.

A useful thing to remember is that Icelandic often follows a verb-second pattern: if something other than the subject comes first, the finite verb usually still stays in the second position.

So:

• Margir | eru | mjög þreyttir á mánudögum
• Á mánudögum | eru | margir mjög þreyttir

Both are grammatical.

No article is needed here.

Icelandic often makes general statements without any article, just as English does:

• Many people are tired on Mondays
• Margir eru mjög þreyttir á mánudögum

The sentence is talking about people and Mondays in a general sense, not about a specific group already identified in the conversation.