Maginn minn er ekki góður í dag.

Maginn is magi (stomach) plus the suffixed definite article -inn, so it literally means the stomach.

In Icelandic, the definite article is often attached to the end of the noun instead of being a separate word like English the.

• magi = stomach
• maginn = the stomach

So maginn minn is literally the stomach my, which is how Icelandic normally expresses my stomach in this kind of sentence.

In Icelandic, possessives like minn (my) often come after the noun when the noun is definite.

So:

• maginn minn = my stomach
• literally: the-stomach my

This is a very normal pattern in Icelandic. English speakers often expect the possessive to come first, but Icelandic frequently places it after the noun.

Because Icelandic usually uses the definite form with this kind of possessive phrase.

So the normal expression is:

• maginn minn = my stomach

Using magi minn without the article is generally not the standard everyday form here. A learner should treat definite noun + possessive as a very common pattern.

Góður is the masculine singular nominative form of góður (good), and it matches maginn.

Icelandic adjectives agree with the noun they describe in:

• gender
• number
• case

Here, maginn is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative:

• góður

Even though góður comes after er, it still agrees with maginn.

Because in a normal Icelandic main clause, ekki usually comes after the finite verb.

So:

• Maginn minn er ekki góður í dag.

This word order is very typical:

• subject
• verb
• ekki
• rest of the sentence

English speakers often want to place not differently, but Icelandic regularly puts ekki after the conjugated verb in main clauses.

Í dag means today.

Literally, í often means in, so the phrase is historically like in the day / on this day, but you should learn í dag as a fixed everyday expression meaning today.

Examples:

• í dag = today
• í morgun = this morning / in the morning
• í kvöld = tonight / this evening

It is best to memorize í dag as a whole phrase.

It is natural Icelandic, especially if you mean that your stomach feels bad or upset today.

It does not usually mean your whole body is sick. It is specifically about the stomach.

So this sentence is natural for something like:

• an upset stomach
• stomach pain
• nausea
• indigestion

If you mean I don’t feel well today in a general sense, Icelandic would more naturally use something like Mér líður ekki vel í dag.

That is simply part of how Icelandic grammar works. Possession is often expressed with:

• a definite noun
• followed by a possessive

So where English says:

• my stomach

Icelandic commonly says:

• maginn minn
• literally: the stomach my

This is one of the patterns English speakers need to get used to rather than translate word for word.

The dictionary forms are:

• magi = stomach
• góður = good

In the sentence:

• maginn is an inflected form of magi
• góður happens to already be in its dictionary form here, because the masculine singular nominative form is also góður

So if you look these up, search for magi and góður.

Yes, that would also be possible, but it means something slightly different in tone.

• Maginn minn er ekki góður í dag = my stomach is not good today / my stomach feels bad today
• Maginn minn er ekki í lagi í dag = my stomach is not okay today

Both can work, but ekki góður is a very natural, simple way to describe that your stomach feels bad.

No. Minn changes to agree with the noun, just like adjectives do.

Here it is minn because maginn is:

• masculine
• singular
• nominative

With other nouns, the form would change. For example, the possessive my can appear in different forms such as:

• minn
• mín
• mitt

and also other case forms.

So minn is not a fixed word in all situations; it is an inflected form that matches the noun.

A rough learner-friendly guide would be:

MAH-yin minn er EHK-ki GOH-thur ee dowg

A few important points:

• g in maginn is not pronounced like a strong English g here
• ll / nn / gi combinations in Icelandic often sound different from what an English speaker expects
• ð in góður is like a soft voiced th in this
• í is a long ee sound

Pronunciation varies a little by speaker, but the main thing is to avoid reading everything with English sound rules.