Learn Hardcore Icelandic: Afmælið mitt er í næsta mánuði.

-ið is the definite article attached to a neuter noun. afmæli = birthday; afmælið = the birthday.
Icelandic often keeps the definite article even when a possessive is present, so afmælið mitt literally means “the birthday my,” i.e., “my birthday.”

Why is the possessive mitt after the noun? Can it go before?

With a definite noun (one that has the article attached), the possessive normally comes after: afmælið mitt.
You cannot say mitt afmælið (preposed possessive + definite suffix) — that’s ungrammatical.
If you drop the definite suffix, you can put the possessive before: mitt afmæli. This can add emphasis/contrast: “MY birthday (as opposed to someone else’s) is next month.” Both afmælið mitt and mitt afmæli are possible, but afmælið mitt is the most neutral here.

Why is it í næsta mánuði and not something with á?

For months, Icelandic idiomatically uses í: í janúar, í febrúar, í næsta mánuði.
For days of the week and many specific dates, you use á: á mánudaginn (on Monday).
For years, you typically use á: á næsta ári (next year). So “in next month” is literally í næsta mánuði.

What case is næsta mánuði, and why those endings?

í takes the dative when it means “in/inside (time/place).”
mánuði is the dative singular of the masculine noun mánuður (month).
næsta is the weak dative masculine singular form of the adjective næstur (next/nearest). Superlatives and “ordinal-like” adjectives (e.g., “first/next/last”) are usually weak when used before a noun, hence næsta mánuði.
Contrast:
- Nominative (as a subject): næsti mánuður (the next month)
- Dative after í: í næsta mánuði

Could I drop í and say Afmælið mitt er næsta mánuð?

The “prepositionless time accusative” (e.g., Næsta mánuð fer ég…) is common with action verbs.
With the copula er (“is”), native usage strongly prefers the preposition: say Afmælið mitt er í næsta mánuði. Without í sounds odd here.

Can I front the time phrase, like “Next month, my birthday is…”?

Yes. Icelandic main clauses are verb-second. You can say:

Í næsta mánuði er afmælið mitt. This is perfectly natural and just shifts emphasis to the time frame.

What’s the literal word-by-word sense of the sentence?

Afmælið (the-birthday) mitt (my) er (is) í (in) næsta (next) mánuði (month.DAT). So literally: “The birthday my is in next month.”

Why is it mitt and not mín?

The possessive agrees with the grammatical gender, number, and case of the noun, not with the speaker.
afmæli is neuter singular nominative here, so you use mitt (neuter). The basic nominative forms are:
- masculine: minn
- feminine: mín
- neuter: mitt

Is there a difference between er and verður in this context?

er (“is”) states the fact that the date falls next month: Afmælið mitt er í næsta mánuði.
verður (“will be/become”) often highlights the occurrence as an event (e.g., the celebration): Afmælið mitt verður í næsta mánuði can imply “The (party/celebration) will be next month.”

What’s the difference between afmæli and afmælisdagur?

afmæli = birthday, the fact/occasion/celebration. Very common in everyday speech for “birthday.”
afmælisdagur = birthday day/date specifically. You can say Afmælisdagurinn minn er í næsta mánuði, but most people still just say afmæli unless they need to stress the exact date.

How does the preposition í work with time in general?

í + dative = “in/during” a period: í næsta mánuði, í þessari viku.
í + accusative can express duration: Hann var þar í mánuð (He was there for a month). So the dative in your sentence marks being “within” that time frame.

Any quick pronunciation tips for this sentence?

æ sounds like “eye” (ai).
á sounds like the “ow” in “cow,” but long and smooth.
í is like long “ee.”
ð (the letter eth) is a soft “th” as in “this,” especially between vowels: -ið, mánuði.
Double consonants like -tt- in mitt are held a bit longer. A rough guide: Af-mæ-lið MITT ehr ee NAI-sta MAO-nu-thi.