Learn Hardcore Finnish: Otan laturin mukaan, jotta kännykkä ei sammu. - I take the charger with me, so that the mobile phone does not turn off.

Finnish usually drops subject pronouns because the verb ending already shows the person. Otan has the 1st-person singular ending -n, so it means “I take.” You can add Minä otan for emphasis or contrast (e.g., “I (as opposed to someone else) will take the charger.”), but it isn’t required.

What case is laturin, and why isn’t it just laturi?

Laturin is the total object in the genitive case. In Finnish:

Affirmative, completed action → total object (singular) in genitive: Otan laturin mukaan.
Negative → partitive object: En ota laturia mukaan.
Ongoing/indefinite amount → partitive: Otan vettä mukaan. (For countable, single items like a charger, partitive for “some of a charger” doesn’t make sense.) Note: In a positive 2nd-person imperative, the total object is nominative: Ota laturi mukaan!

Does mukaan force the genitive case on laturin?

Not here. In this construction, laturin is the verb’s object, and its case follows object rules (see above). The adverb/postposition mukaan means “along/with (me)” in the set phrase ottaa X mukaan “take X along.” You can see it’s the object’s case, not mukaan’s, from the alternative word order: Otan mukaan laturin (still genitive because it’s a total object).
Separately, when mukaan means “according to,” it does require a genitive: ohjeiden mukaan “according to the instructions.”

Can I say Otan laturin kanssa instead of Otan laturin mukaan?

No. kanssa means “with (together with)” in the sense of accompaniment: Menin ystävän kanssa “I went with a friend.”
Ottaa X mukaan is the idiomatic way to say “take X along/with me.” Otan laturin kanssa would suggest “I take (something) with the charger,” which doesn’t say you’re taking the charger itself.

Is the word order fixed? Can I say Otan mukaan laturin?

Both Otan laturin mukaan and Otan mukaan laturin are fine. Placing mukaan before or after the object is common. Word order can lightly affect emphasis/focus, but both are neutral here.

What exactly does jotta mean, and how is it different from että?

jotta introduces a purpose clause: “so that / in order that.”
että generally means “that” (content clause) or can introduce results.
In purpose clauses, jotta (or its negative forms) is the standard choice: Otan laturin mukaan, jotta… “I’ll take the charger so that…”. In everyday speech, many people use että with a negative (ettei), but in careful/standard style jotta (ei) or jottei is preferred for purpose.

Can I say jottei or ettei instead of jotta ei?

Yes:

jotta ei = “so that … not” (explicit, standard)
jottei = contracted form of jotta ei (very common, also standard)
ettei = “that … not” (from että ei). Many speakers use it in purpose clauses in speech, but for clear purpose, jotta ei / jottei is stylistically safer.
Examples: … jotta kännykkä ei sammu, … jottei kännykkä sammu, … ettei kännykkä sammu.

Why is it ei sammu and not ei sammuu?

In the negative present, Finnish uses a special “connegative” verb form after the negative auxiliary ei. The positive 3rd person singular is sammuu (“it turns off”), but the connegative form is sammu: kännykkä ei sammu (“the phone doesn’t turn off”).
Pattern: negative auxiliary (ei) + connegative stem (sammu).

Why present tense for a future meaning? Why not a “future tense”?

Finnish has no separate future tense. The present is used for present and future. … jotta kännykkä ei sammu naturally means “so that the phone won’t turn off.” Context supplies futurity.

What’s the difference between sammua and sammuttaa?

sammua = intransitive, “to go out/turn off (by itself).” → Kännykkä sammuu “The phone turns off.”
sammuttaa = transitive, “to turn off (something).” → Sammutan kännykän “I turn off the phone.”
In your sentence, the phone is the subject of an intransitive verb, so sammua is correct: … ettei se sammu.
Tip: Imperative of sammuttaa is Sammuta! (“Turn it off!”), which looks similar but is a different verb.

Is kännykkä the normal word for “phone”? Could I use puhelin?

kännykkä is everyday/colloquial for “mobile/cell phone.” puhelin is the general word for “telephone” and is more neutral/formal. Both are fine: jotta puhelin ei sammu is perfectly correct; kännykkä just feels more casual.

Why is there a comma before jotta?

In Finnish punctuation, when a main clause comes first and is followed by a subordinate clause introduced by a conjunction (like jotta, että, koska, vaikka), you put a comma before the conjunction: Otan laturin mukaan, jotta …
If the subordinate clause comes first, there’s also a comma between the clauses: Jotta kännykkä ei sammu, otan laturin mukaan.

Could I talk about the battery instead, like “so the battery doesn’t run out”?

Yes. Natural options:

… jotta akku ei lopu (“so that the battery doesn’t run out”)
… jotta akku ei tyhjene (“so that the battery doesn’t empty”)
All are idiomatic. Your original … jotta kännykkä ei sammu focuses on the phone shutting down, which is what you’re trying to avoid by taking a charger.

What if I want to say “bring” instead of “take”? Should I use tuoda?

ottaa = take (pick up, take along with you)
tuoda = bring (toward the speaker/listener’s location)
viedä = take (to some other location, away from here)
Examples:
To someone at the destination: Tuon laturin mukaan “I’ll bring the charger (with me to you/us).”
Neutral “take along”: Otan laturin mukaan.
To emphasize taking it away somewhere: Vien laturin mukaan (less common phrasing; often just Vien laturin).

How does the object change if I negate the main clause?

It switches to partitive:

Affirmative: Otan laturin mukaan.
Negative: En ota laturia mukaan.
The purpose clause can still be affirmative or negative independently, but semantically your example makes sense only if you take the charger.

How do numerals affect the object? For example, “I’ll take two chargers along.”

With numerals, the noun is in the partitive singular after the number:

Otan kaksi laturia mukaan. (“I’ll take two chargers along.”)
Otan kolme laturia mukaan.
This is a general rule: numerals 2+ → partitive singular of the noun.

How would this look in the impersonal (the “passive”)? For example, “Shall we take the charger along?”

Question (impersonal): Otetaanko laturi mukaan?
In the impersonal, a total object appears in the nominative (not genitive): laturi, not laturin.
Answering: Otetaan laturi mukaan. / Ei oteta laturia mukaan. (Negative switches the object to partitive.)

Can I replace kännykkä with a pronoun in the second clause?

Yes, if the referent is clear: … jotta se ei sammu (“so that it doesn’t turn off”). Finnish often repeats the noun for clarity, but se is fine in context.

Is there any ambiguity with mukaan meaning “according to”?

Context and verb choice remove the ambiguity. With ottaa, mukaan means “along.” “According to” uses mukaan with a genitive NP that makes sense as a source/authority: ohjeiden mukaan (“according to the instructions”). laturin mukaan as “according to the charger” would be odd, so here mukaan can only mean “along.”