Learn Hardcore Finnish: Ellei tänään sada, kävelen metsään yksin. - Unless it rains today, I will walk into the forest alone.

Ellei means “unless / if … not”. It’s essentially a fused form of ellen + ei historically, but for learners it’s easiest to treat it as a single conjunction meaning “if not”.
It introduces a negative condition, so you don’t use ei separately: you say ellei … rather than ellei ei ….

Could I replace ellei with jos ei? Is there any difference?

Yes. Jos ei tänään sada, … means the same basic thing: “If it doesn’t rain today, …”
Typical differences:

ellei is a bit more compact and often feels slightly more formal/literary.
jos ei is very common in everyday speech and is very transparent for beginners.

Both are correct here.

Why is the verb sada in the form sada (not sataa or something else)?

The dictionary form is sataa = to rain.
But after the negative/conditional structure here, Finnish uses the connegative form, which looks like the verb stem without personal endings. For sataa, the connegative is sada.

So:

Positive: Tänään sataa. = “It’s raining today.”
Negative: Tänään ei sada. = “It isn’t raining today.”
With ellei: Ellei tänään sada, … = “If it doesn’t rain today, …”

Why isn’t there a subject for sada?

Weather verbs like sataa are used impersonally in Finnish—similar to English “it rains”, but Finnish typically doesn’t use a dummy subject like it.
So you just say sataa / ei sada without an explicit subject.

Why is there a comma after sada?

Finnish normally uses a comma between a subordinate clause and the main clause.
Here, Ellei tänään sada is the conditional subordinate clause, and kävelen metsään yksin is the main clause, so a comma is standard:

Ellei tänään sada, kävelen …

(You’ll see the same with jos, kun, vaikka, etc.)

Why does kävelen look like present tense if the meaning is future (“I will walk”)?

Finnish often uses the present tense to talk about future plans or intentions when the context makes it clear. The conditional clause sets up the future situation, so kävelen naturally reads as “I’ll walk / I’m going to walk”.

If you want to emphasize intention or planned future, you can also use other constructions, but plain present is very normal:

Huomenna menen töihin. = “Tomorrow I’m going to work.”

Should it be kävelen or kävelisin (conditional mood)?

Both can work, but they suggest slightly different attitudes:

Ellei tänään sada, kävelen metsään yksin.
Sounds like a straightforward plan: “If it doesn’t rain today, I’ll walk…”
Ellei tänään sada, kävelisin metsään yksin.
More tentative/hypothetical, like “I’d walk…” (less committed, more conditional-sounding in English too).

In your sentence, kävelen fits a clear intention.

What case is metsään, and why that one?

Metsään is the illative case, meaning movement into something: “into the forest.”

metsä = forest
illative singular: metsään

Contrast:

metsässä (inessive) = in the forest (location, no movement)
metsästä (elative) = out of the forest

So kävelen metsään focuses on going into the forest.

Why is yksin used instead of something like “with no one” or a case form?

Yksin is a very common adverb meaning “alone”. It modifies the verb phrase: “I walk … alone.”
You can also say things like:

yksinäni = “alone (by myself)” (more emphatic / stylistic) But yksin is the neutral, everyday choice.

Is the word order fixed? Could I move tänään or yksin?

Finnish word order is flexible, but changes often add emphasis.

Neutral/common:

Ellei tänään sada, kävelen metsään yksin.

Possible variations:

Ellei sada tänään, … (slightly different rhythm; can emphasize today by moving it)
Ellei tänään sada, kävelen yksin metsään. (puts a bit more focus on alone earlier)

All are grammatical; the “best” depends on what you want to highlight.

How would I say “If it doesn’t rain today, I’ll walk to the forest alone” (not “into the forest”)?

If you mean “to the forest” as reaching it (toward it), you’d typically use metsälle (allative) in many contexts:

Ellei tänään sada, kävelen metsälle yksin.

But nuances vary by what you mean (to the forest area vs into the woods). In many real situations, metsään is the most natural if you actually go among the trees.