Unohdin sammuttaa herätyskellon, joten heräsin kahdesti.

After a verb like unohtaa (“to forget”), if you want it to take another verb as its object, you use the first infinitive (–a/–ä form): sammuttaa.
The form sammuttamaan is the third infinitive inessive or illative (used for indicating purpose, like “in order to turn off”), so it doesn’t work as the direct object of unohtaa.

In Finnish, transitive verbs often take a genitive object when the action is viewed as completed or specific.
Here, sammuttaa (“to turn off”) + herätyskello (“alarm clock”) → sammuttaa herätyskellon expresses that you actually turned the alarm clock off (a definite, completed action).

Finnish is a pro-drop language: the verb endings already encode person and number.
– unohdin = “I forgot” (first-person singular ending -n)
– heräsin = “I woke up” (first-person singular ending -in)
Since the verbs clearly show “I,” you usually omit minä unless you need extra emphasis.

They look similar but have opposite valency:
• herätä (intransitive) = “to wake up” (yourself or to wake naturally)
– e.g. heräsin kahdesti = “I woke up twice.”
• herättää (transitive) = “to wake somebody (else) up”
– e.g. herätän sinut kuudelta = “I’ll wake you at six.”

In Finnish, you always put a comma before coordinating conjunctions like ja, mutta, joten, sillä etc., when they connect two independent clauses: “Clause 1, joten Clause 2.”

Both mean “twice,” but:
• kahdesti is an adverb formed with ‑sti from kaksi. It’s more formal and concise.
• kaksi kertaa literally means “two times” and uses the numeral kaksi + partitive kertaa (“time”). It’s perfectly correct and maybe more colloquial.

That exact inversion wouldn’t make logical sense (you can’t wake up twice before forgetting to turn it off!).
But you can generally swap main clauses in Finnish, keeping the comma+conjunction:
“Heräsin kahdesti, joten unohdin sammuttaa herätyskellon” → meaning changes to “I woke up twice, so I forgot to turn off the alarm,” which reverses cause and effect and sounds odd here.