Learn Hardcore Icelandic: Hún er ósammála mér.

In English, you say disagree with someone. In Icelandic, the idea of with is built into the adjective ósammála, which governs the dative case instead of using a preposition:

ósammála + dative = disagree with

So Icelandic just says:

Hún er ósammála mér. – literally “She is disagreeing-to-me” (using dative), no extra word for with needed.

Is ósammála a verb or an adjective?

Ósammála is an adjective, not a verb. The verb in the sentence is er (the 3rd person singular of vera – to be).

The structure is:

Hún (she) – subject, nominative
er (is) – verb
ósammála (in disagreement) – predicative adjective
mér (to me) – dative object of the adjective

So the pattern is: subject + vera (to be) + sammála/ósammála + dative.

Does ósammála change form for gender or number, like other adjectives do?

No. Sammála and ósammála are indeclinable adjectives in modern usage. They:

Do not change for gender (masculine/feminine/neuter),
Do not change for number (singular/plural),
Do not change for case.

You say:

Ég er ósammála. – I disagree.
Hann er ósammála. – He disagrees.
Hún er ósammála. – She disagrees.
Þau eru ósammála. – They (neuter/pl.) disagree.

The form ósammála stays exactly the same.

Can I say Hún er ekki sammála mér instead? Is there a difference?

Both are grammatical, but there is a nuance:

Hún er ekki sammála mér. – She does not agree with me.
- Neutral negation of agreement; often slightly softer.
Hún er ósammála mér. – She disagrees with me.
- More direct; can feel a bit stronger or more definite.

In many everyday contexts they can overlap, but ósammála is slightly more emphatic about the disagreement itself.

Can I leave out mér and just say Hún er ósammála?

Yes, if the context makes it clear who she disagrees with.

Hún er ósammála. – She disagrees / She doesn’t agree (with what was just said or with someone already obvious from context).

If you want to specify the person and avoid ambiguity, you include the dative:

Hún er ósammála mér. – She disagrees with me.
Hún er ósammála honum. – She disagrees with him.

Can I change the word order, like Mér er hún ósammála?

Yes, but with some limits.

The neutral, most common word order is:
Hún er ósammála mér.

You can front mér for emphasis:

Mér er hún ósammála. – It’s me she disagrees with. (emphasizing mér)

But you cannot freely scramble everything. For example:

*Hún ósammála er mér – is not natural Icelandic.

The safe patterns to copy are:

Hún er ósammála mér. (neutral)
Mér er hún ósammála. (emphatic, but correct)

How would I say “I disagree with her” using the same pattern?

Follow the same structure: subject + er + ósammála + dative.

Ég er ósammála henni. – I disagree with her.

Forms to notice:

Ég – nominative (I, subject)
henni – dative of hún (she → to her)

What case do nouns take after ósammála?

Nouns after sammála / ósammála also go in the dative:

Ég er ósammála kennaranum. – I disagree with the teacher.
Við erum sammála foreldrunum. – We agree with the parents.
Hún er ósammála yfirmanninum. – She disagrees with the boss.

So the rule is: sammála / ósammála + dative (pronoun or noun).

Can ósammála be used in front of a noun like a normal adjective, e.g. “the disagreeing woman”?

Very rarely, and not in normal, everyday Icelandic. Sammála / ósammála are practically used only as predicative adjectives after vera:

Hún er ósammála mér. – normal.

Using it attributively (before a noun), like:

*hin ósammála kona – the disagreeing woman

would sound strange or ungrammatical in ordinary speech. If you want to say the woman who disagrees, you’d usually rephrase:

Konan sem er ósammála. – The woman who disagrees.

How formal or informal is the sentence Hún er ósammála mér?

It’s neutral and standard. You can use it:

In casual conversation,
In writing (emails, articles),
In fairly formal contexts.

It does not sound slangy or overly formal. It’s simply the normal way to say “She disagrees with me” in Icelandic.