Learn Hardcore Icelandic: Hann lærði af mistökunum sínum.

In this idiom you use the preposition af: the pattern is læra af e-u (to learn from something). Af often means “off of” or “as a result of,” which fits the idea of drawing lessons from something. Frá is more about physical origin or source (from a place/person).

Natural: Hann lærði af mistökunum sínum.
Unnatural for this meaning: Hann lærði frá mistökunum sínum. Related contrasts:
læra af honum = learn from him (by observing his example)
læra hjá honum = study under him (be his student)
sögur frá honum = stories from him (he told them)

What does sínum mean, and why not hans?

Sínum is the reflexive possessive (dative plural of sinn), referring back to the subject of the clause. Because the subject is Hann, sínum means “his own.”

Hann lærði af mistökunum sínum. = He learned from his own mistakes.
Hann lærði af mistökunum hans. = He learned from his (another man’s) mistakes. Use sinn/sín/sitt only for 3rd-person “own.” For 1st/2nd person, use the regular possessives:
Ég lærði af mistökunum mínum.
Þú lærðir af mistökunum þínum.

Why is mistökunum in the dative plural, and what is the -unum ending?

Af governs the dative, so the noun after it must be dative.
Mistök is neuter plural. Dative plural (indefinite) is mistökum.
With a postposed possessive (sínum), the noun usually takes the definite article, so you get the definite dative plural: mistökunum. Morphology:
Base plural: mistök
- definite article (pl.): mistökin
- dative plural ending: mistökunum (= mistök + in + um)

Does mistök have a singular?

No. Mistök is a plural-only (pluralia tantum) neuter noun meaning “mistake(s).”

Nom/Acc pl.: mistök
Dat pl.: mistökum
Gen pl.: mistaka
Definite dative pl.: mistökunum

Can I say af sínum mistökum instead of af mistökunum sínum?

Yes. Both are grammatical:

Postposed possessive (more neutral in style, noun takes the definite article): af mistökunum sínum
Preposed possessive (a bit more contrastive/emphatic, noun is usually indefinite): af sínum mistökum

Could I leave out the possessive and just say Hann lærði af mistökunum?

Grammatically yes, but you then say “He learned from the mistakes,” without specifying whose. If you want to make it clear they are his own, keep sínum (or use hans if you mean another man’s mistakes).

What tense is lærði, and how do I conjugate læra?

Lærði is the simple past (preterite) of læra (“to learn”).

Present: ég læri, þú lærir, hann lærir
Past: ég lærði, þú lærðir, hann lærði
Perfect: ég hef lært, hann hefur lært Note: lært is the supine used in perfect tenses; lærður is an adjective meaning “learned/educated.”

Why Hann and not Honum?

Hann is nominative (subject case). Honum is dative (“to/for him”) and is used after certain prepositions or verbs, e.g., af honum (“from him”), honum líkar (“he likes/it pleases him”).

Any pronunciation tips for this sentence?

Hann: short “a” as in “hut,” clear double n (a longer n).
lærði: æ like the diphthong in “eye”; ð like the “th” in “this”; final i like a short “i” in “bit.” Roughly “LAI-r-thi.”
af: pronounced with a v-sound at the end: “av.”
mistökunum: ö like German “ö” (rounded “eh”); stress on the first syllable: “MIS-tœ-kun-um.”
sínum: í is a long “ee”; stress on the first syllable: “SEE-num.”

Is af always followed by the dative?

Yes. Af takes the dative:

af borðinu (off the table)
af honum (from him)
af mistökunum (from the mistakes)

What’s the word order rule here? Can I front the prepositional phrase?

Icelandic main clauses are verb-second (V2). The finite verb sits in position 2, regardless of what comes first.

Neutral: Hann lærði af mistökunum sínum.
Fronted PP (focus on the “from…” part): Af mistökunum sínum lærði hann.

Could I use úr or vegna instead of af?

Different meanings:

úr = “out of/from inside” (physical or figurative extraction). Not idiomatic with læra here.
vegna (+ genitive) = “because of/due to.” That states cause, not the idea of drawing lessons. You could say:
- Hann lærði af mistökunum sínum (he drew lessons from them)
- Hann lærði vegna mistaka sinna (he learned as a result of his mistakes), but this is less idiomatic for the intended sense.

What’s the gender and declension info I need for mistök?

Gender/number: neuter plural-only.
Key forms: mistök (nom/acc pl.), mistökum (dat pl.), mistaka (gen pl.).
With a postposed possessive, add the definite article to the noun: mistökunum sínum.

How would this look with other persons or subjects?

1st sg.: Ég lærði af mistökunum mínum.
2nd sg.: Þú lærðir af mistökunum þínum.
1st pl.: Við lærðum af mistökunum okkar.
3rd sg. fem.: Hún lærði af mistökunum sínum. Remember: only 3rd person uses the reflexive sinn/sín/sitt to mean “own.”

Are there alternatives to mistök?

Yes, villa (f.) “error, mistake” is common:

Plural: villur
Dative plural definite: villunum
Example: Hann lærði af villunum sínum. (He learned from his errors.)