Afleiðingin er mikil.

It is made of:

• afleiðing = consequence, result
• -in = the feminine singular definite article, attached to the noun

So afleiðingin means the consequence or the result.

The dictionary form is afleiðing.

In Icelandic, the definite article is usually a suffix, not a separate word like English the.

So:

• afleiðing = a consequence / consequence
• afleiðingin = the consequence

This is very normal in Icelandic. Many nouns work this way:

• bíll = car

• bíllinn = the car

• kona = woman
• konan = the woman

Afleiðing is a feminine noun.

That matters because adjectives in Icelandic must agree with the noun they describe or refer back to. In this sentence, mikil agrees with afleiðingin in:

• gender: feminine
• number: singular
• case: nominative

That is why you get mikil, not some other form.

Because mikill changes form depending on gender, number, and case.

Here are the nominative singular forms:

• mikill = masculine
• mikil = feminine
• mikið = neuter

Since afleiðingin is feminine singular, the correct form is mikil.

So:

• bíllinn er mikill would be masculine
• afleiðingin er mikil is feminine
• húsið er mikið would be neuter

Here mikil does not usually mean physically big. With an abstract noun like afleiðing, it means something more like:

• great
• considerable
• significant
• serious (depending on context)

So the sentence suggests that the consequence is substantial or important, not literally large in size.

Both are in the nominative singular feminine.

Why?

• afleiðingin is the subject of the sentence, so it is in the nominative.
• After vera (to be), a predicate adjective like mikil usually also appears in the nominative and agrees with the subject.

So the grammar is:

• Afleiðingin = nominative singular feminine
• er = is
• mikil = nominative singular feminine

Because Icelandic verbs do not agree with gender. They agree mainly with person and number.

Er is the 3rd person singular form of vera (to be), so it is used with he/she/it and singular nouns:

• Maðurinn er... = The man is...
• Konan er... = The woman is...
• Húsið er... = The house is...

The noun’s gender changes the adjective, not the verb.

Yes. If you want to say a consequence is great/significant, you would usually use:

• Afleiðing er mikil.

But in real usage, Icelandic often prefers a fuller structure or some context, because a bare singular noun can sound more formal, general, or incomplete depending on the situation.

The key contrast is:

• afleiðing = consequence
• afleiðingin = the consequence

No, this is the basic and very common word order:

• Subject + verb + complement
• Afleiðingin + er + mikil

So it works just like English The consequence is significant.

Icelandic does allow other word orders for emphasis, especially in more literary or formal language, but this sentence is the straightforward neutral order.

A rough English-friendly guide is:

• Afleiðingin ≈ AV-lay-thing-in
• er ≈ ehr
• mikil ≈ MIH-kil

A few useful notes:

• af is usually pronounced more like av
• ei sounds like ay
• ð in leið is a soft voiced sound, like th in this
• the g in -ing- is not pronounced like a hard English g

A more careful pronunciation varies a bit by speaker, but that rough guide will get you close.

Yes, it is grammatical and natural.

It sounds like something you might say in a formal, explanatory, or written context, especially when talking about the seriousness or scale of an outcome.

Depending on context, Icelandic might also use other adjectives for more specific meanings, for example:

• Afleiðingin er alvarleg. = The consequence is serious.
• Afleiðingin er veruleg. = The consequence is considerable/substantial.

But Afleiðingin er mikil is perfectly valid and understandable.