Learn Hardcore Finnish: Lisään öljyä ja sekoitan kaiken, sitten maustan suolalla. - I add oil and mix everything, then season with salt.

Öljyä is the partitive singular of öljy. With mass nouns and when you mean an indefinite amount (“some oil”), Finnish uses the partitive object.
With lisätä (“to add”), the thing being added is typically in the partitive if the amount isn’t delimited: Lisään öljyä = “I add (some) oil.”
Öljyn would mean “the oil” as a whole, a specific, bounded amount already known in the context: Lisään öljyn (“I add the oil [that we measured/mentioned]”). That’s less natural in a recipe unless you’re referring to a specific pre‑measured portion.
Bare öljy isn’t used as a direct object; objects are marked (here by partitive/genitive/nominative depending on context).

What is kaiken, and why not kaikki or kaikkea?

Kaiken is the genitive singular form of kaikki (“everything”). As an object, this -n form functions as the total object (accusative) in the singular. Sekoitan kaiken = “I mix all of it (everything).”
Kaikkea is the partitive singular (“some of everything” / unbounded), which would suggest an incomplete/indefinite object — odd here.
Kaikki (nominative) is used as a subject (“Kaikki on hyvin” = “Everything is fine”) or as a determiner with a plural noun: Sekoitan kaikki ainekset (“I mix all the ingredients”).

Why is suolalla in -lla? What case is that and what does it mean?

Suolalla is the adessive case (ending -lla/-llä). One of its common uses is to mark the instrument/means: “with/by means of.”
With maustaa (“to season”), the spice/herb is usually in the adessive: maustaa X:llä = “season with X.”
Here the food being seasoned is understood from context (the mixture), so it’s omitted: (Sekoitan kaiken,) sitten maustan suolalla.

Could I say maustan suolaa or use suolaan instead?

Maustan suolaa would literally be “I season the salt” — not what you want.
To express adding salt as a seasoning, either say maustan suolalla (“I season with salt”) or simply lisään suolaa (“I add some salt”).
Suolata is a separate verb meaning “to salt” (often more literally/heavily): Sitten suolaan (sen). In everyday cooking, maustaa suolalla or lisätä suolaa is more common.

Why is there a comma before sitten? Could I write ja sitten or use a period instead?

Finnish typically separates independent main clauses with a comma if they aren’t joined by a conjunction. Here, after the first clause, you start a new main clause with sitten, so the comma is natural.
Alternatives:
- Lisään öljyä ja sekoitan kaiken. Sitten maustan suolalla.
- Lisään öljyä ja sekoitan kaiken, ja sitten maustan suolalla.
- All are acceptable; pick the rhythm you like.

Is sitten necessary?

Not strictly. It’s a temporal adverb (“then”) that clarifies sequence. You can drop it if the order is obvious:
- Lisään öljyä ja sekoitan kaiken, sitten maustan suolalla.
- Lisään öljyä ja sekoitan kaiken, maustan suolalla.
Synonyms: sen jälkeen (“after that,” a bit more formal), or colloquial sit.

Where is the subject? Why isn’t minä written?

Finnish is “pro-drop”: the verb ending shows the person/number, so the subject pronoun is usually omitted.
Lisään / sekoitan / maustan all end in -n (1st person singular), so minä is understood. You can add Minä for emphasis/contrast.

How are these verbs formed in the present? And why does sekoitan have only one t?

1st person singular present ends in -n:
- lisätä → lisään
- sekoittaa → sekoitan
- maustaa → maustan
The tt → t change in sekoitan is regular consonant gradation (compare ottaa → otan). Many -ttaa verbs show this in finite forms.

What’s the difference between kaiken and kaikki ainekset?

Kaiken = “everything (all of it)” as a pronoun, total object (singular).
Kaikki ainekset = “all the ingredients,” where kaikki is a determiner and ainekset is a plural noun (and as a total object, the plural stays nominative: sekoitan kaikki ainekset).

Why do we see ä and ö here? Does vowel harmony matter?

Finnish has vowel harmony. Words with front vowels (ä, ö, y) take front-vowel endings.
Lisään uses ä (from lisätä) and öljyä uses ä in the ending because öljy contains front vowels (ö, y).
Suola has back vowels (u, o, a), so its adessive is -lla: suolalla (not suolällä).

How do you pronounce öljyä?

Rough guide:
- ö ≈ French “eu” (in “bleu”) or German “ö”.
- y ≈ French “u” (in “lune”) or German “ü”.
- ä ≈ “a” in “cat.”
Syllables: ölj-y-ä. Say the lj cluster smoothly: [öl-jy-ä]. All vowels are short here.

Could I change the order of the actions?

Yes, Finnish word order is flexible, but it affects emphasis/sequence. Without sitten, swapping clauses can imply a different order:
- Sekoitan kaiken ja lisään öljyä suggests you mix first, then add oil.
- Using sitten or a period helps keep the timeline clear: Lisään öljyä ja sekoitan kaiken. Sitten maustan suolalla.

When would öljyn be correct instead of öljyä?

When referring to a specific, bounded amount known to speaker and listener:
- Lisään öljyn seokseen = “I’ll add the oil (the specific portion we talked about) to the mixture.”
In recipes and general instructions, the indefinite öljyä is far more common.