Learn Hardcore German: Wir gehen spazieren, sofern es nicht regnet. - We will go for a walk, provided it doesn’t rain.

wenn is the everyday, general "if/when" and fits almost everywhere.
falls suggests a mere possibility ("in case"); it's common in spoken German.
sofern means "provided that" and sounds a bit more formal or written. Your sentence works with all three; choose based on register and nuance.

Why is there a comma before sofern?

Because sofern introduces a subordinate clause. In German, every subordinate clause is set off by a comma from the main clause: Wir gehen spazieren, sofern es nicht regnet.

Why does the verb go to the end in sofern es nicht regnet?

Subordinating conjunctions like sofern send the finite verb to the end of the clause. Hence: … sofern es nicht regnet (verb-final), not … sofern es regnet nicht.

Why is it nicht regnet and not regnet nicht?

In a subordinate clause, the finite verb must appear at the end, and nicht precedes it: … es nicht regnet. Regnet nicht is fine in a main clause (Es regnet nicht.), but not after a subordinating conjunction.

Can I start with the condition instead: Sofern es nicht regnet, gehen wir spazieren?

Yes. When the subordinate clause comes first, the finite verb of the main clause stays in second position, so you get inversion: Sofern es nicht regnet, gehen wir spazieren.

Is sofern too formal for everyday speech?

A little. You'll hear wenn more in casual conversation: Wir gehen spazieren, wenn es nicht regnet. Sofern is common in written German, announcements, or careful speech.

Do I need future tense (werden) for a future plan?

No. German often uses the present for future meaning: Wir gehen morgen spazieren, sofern es nicht regnet. You can say Wir werden spazieren gehen, but it's usually unnecessary unless you want to stress futurity or avoid ambiguity.

Why do we need es in sofern es nicht regnet? Can I drop it?

No. Weather verbs like regnen are impersonal and require the dummy subject es: Es regnet. You cannot omit es in standard German.

Could I use kein instead of nicht?

No. Kein negates nouns (or something with an implied noun), while nicht negates verbs, adjectives, or adverbs. Here you're negating the verb regnet, so it's nicht.

Is there any difference between wir gehen spazieren and wir spazieren?

Both are grammatical. Spazieren gehen is the most idiomatic way to say "go for a walk," especially without further detail. Wir spazieren tends to appear with a complement: Wir spazieren durch den Park, and is less common as a standalone plan.

How else can I say this? Is einen Spaziergang machen okay?

Yes: Wir machen einen Spaziergang, sofern es nicht regnet. This is equivalent in meaning and register to wir gehen spazieren.

How does the word order behave with extra info like time and place?

Keep normal German order in the main clause and keep the verb-final rule in the sofern-clause:

Wir gehen morgen im Park spazieren, sofern es nicht regnet.
Sofern es morgen nicht regnet, gehen wir im Park spazieren.

What's the best way to say "unless it rains"?

Use es sei denn: Wir gehen spazieren, es sei denn, es regnet. Your original with sofern … nicht is close in meaning and often interchangeable, but es sei denn is the canonical "unless."

What happens to spazieren gehen inside a subordinate clause?

The verb complex goes to the end: …, dass wir spazieren gehen, sofern es nicht regnet. The two parts stay together at the end of their clause.

Is solange a good substitute here?

Be careful. Solange is temporal ("as long as") and focuses on duration: Solange es nicht regnet, gehen wir spazieren suggests "while it isn't raining, we are walking." Sofern/wenn express a condition, not a time span.