Learn Hardcore German: Wenn irgendjemand Hilfe braucht, melde ich mich sofort. - If anybody needs help, I will get in touch immediately.

Because wenn introduces a subordinate (dependent) clause in German. In subordinate clauses, the finite verb goes to the end: Wenn … braucht. Example: Wenn ich Zeit habe, komme ich.

Why does the main clause start with melde instead of ich?

German main clauses are verb-second (V2). The entire wenn-clause counts as one element in the first position, so the finite verb of the main clause must come next, causing inversion: …, melde ich mich …. Without fronting the wenn-clause, you’d get: Ich melde mich sofort, wenn …

Is the comma required?

Yes. A comma is mandatory before/after a subordinate clause: Wenn …, …. German always marks subordinate clauses with commas.

What’s the difference between wenn, wann, and falls?

wenn: if/whenever/when (conditional or repeated time). Fits here.
wann: when, used in questions or indirect questions (e.g., Ich weiß nicht, wann er kommt.).
falls: in case, a bit more tentative/hypothetical than wenn (e.g., Falls jemand Hilfe braucht, … sounds more like a precaution).

What nuance does irgendjemand add compared to jemand? What about irgendwer?

jemand = someone (unspecified).
irgendjemand = anyone at all / just anyone, emphasizing indeterminacy.
irgendwer = colloquial for anyone/someone; a bit more casual than irgendjemand. All three are singular.

Does irgendjemand take singular or plural, and how does it decline?

It’s always singular. Verb agreement is singular: irgendjemand … braucht (not brauchen). Declension (same pattern as jemand):

Nominative: (irgend)jemand
Accusative: (irgend)jemanden
Dative: (irgend)jemandem
Genitive: (irgend)jemandes (rare)

Why is there no article before Hilfe?

Abstract or mass nouns often appear without an article when used in a general sense. Hilfe here means help in general. You’d use an article if you specify: die Hilfe des Teams, keine Hilfe, ein bisschen Hilfe.

What case is Hilfe, and what governs it?

Accusative, as the direct object of brauchen. brauchen is a transitive verb that takes an accusative object: jemand braucht Hilfe / jemand braucht sie.

Could I use benötigen or helfen instead of brauchen?

benötigen ≈ need/require, more formal: Wenn jemand Hilfe benötigt, …
helfen works differently: it’s intransitive and takes a dative object: Wenn jemand Hilfe braucht, helfe ich ihm/ihr sofort. (not: helfe ich Hilfe)

What exactly does sich melden mean here, and why mich?

sich melden is a reflexive verb meaning to get in touch, report back, speak up, or volunteer. The reflexive pronoun agrees with the subject; for ich, it’s mich (accusative): ich melde mich, du meldest dich, er/sie meldet sich, wir melden uns, ihr meldet euch, sie melden sich.
You can add the recipient with bei + dative: Ich melde mich sofort bei dir/euch/ihnen.

Would … helfe ich sofort be more natural than … melde ich mich sofort?

Both are fine but not identical:

… melde ich mich sofort = I will contact/speak up immediately (to offer help, coordinate, etc.).
… helfe ich sofort = I will help immediately (commits to the act of helping). Typically add a dative object if needed: … helfe ich ihm/ihr sofort.

Where can sofort go? Could I say gleich, direkt, or umgehend?

Best is clause-final or near the end: Ich melde mich sofort.
Alternatives:

gleich can mean right away, but regionally also soon/in a moment (can be ambiguous).
direkt sometimes works for right away, but also means directly/straight (route).
umgehend is formal for promptly.
sofort is the clearest for immediately.

Why is the present tense used? Should it be werde mich melden or würde?

German often uses the present for near-future or general conditions: Wenn …, melde ich mich ….
You may use:

werde mich (sofort) melden for explicit future.
würde mich (sofort) melden for a hypothetical/polite conditional or more tentative tone. All are possible, with slight nuance differences.

Can I add dann to make an if–then structure explicit?

Yes: Wenn irgendjemand Hilfe braucht, dann melde ich mich sofort. dann is optional; it can add emphasis or clarity but doesn’t change the grammar.

Could I rephrase the sentence using wer?

Yes, but adjust the structure:

Wer Hilfe braucht, dem helfe ich sofort. (dative dem refers back to wer)
Wer Hilfe braucht, der kann sich bei mir melden.
Don’t say Wer …, melde ich mich … without a clear reference; use the wer … der/dem … pattern or reword.