Learn Hardcore German: Meine Finger werden warm, sobald sie mit ihren Händen arbeitet. - My fingers become warm as soon as she works with her hands.

Werden with an adjective (warm) expresses a change of state: “werden warm” = “to become/get warm.”
If you said “Meine Finger sind warm”, it would just describe a state: My fingers are warm (already).
“Meine Finger werden warm” focuses on the process: they are starting to get warm.

What exactly does werden mean here? Is it the future tense?

Here werden is not a future-tense auxiliary; it is a full verb meaning “to become” / “to get.”
So “werden warm” literally means “become warm.”
The German future tense would be “Meine Finger werden warm werden” (they will become warm), but that would sound very unusual here.
In everyday German, present tense often covers future meaning, so context decides whether werden is “will” or “become.” Here it clearly means “become.”

Could the sentence also be “Meine Finger sind warm, sobald sie mit ihren Händen arbeitet”?

Grammatically, yes, but the meaning changes.

“werden warm” = they start to get warm when she works with her hands.
“sind warm” = they are (already) warm whenever she works with her hands.
So “werden warm” highlights the change that happens at that moment, which is usually what you want here.

What does sobald mean, and why use it instead of wenn?

Sobald means “as soon as.”
It expresses that one thing happens immediately after another.
You could say “wenn sie mit ihren Händen arbeitet”, which is more like “when(ever) she works with her hands,” but it’s less precise about immediacy.
So sobald is chosen to stress that the fingers get warm right away when she starts working.

What word order does sobald cause in the clause “sobald sie mit ihren Händen arbeitet”?

Sobald introduces a subordinate clause. In such clauses, in German, the finite verb goes to the end.
So you get:

Subject: sie
Other elements: mit ihren Händen
Verb at the end: arbeitet
Thus: sobald sie mit ihren Händen arbeitet (not “sobald sie arbeitet mit ihren Händen”).

Can I put the sobald clause at the beginning of the sentence instead?

Yes. You can also say:
“Sobald sie mit ihren Händen arbeitet, werden meine Finger warm.”
This is very natural in German.
The comma separates the subordinate clause (Sobald…arbeitet) from the main clause (werden meine Finger warm).
In both versions, the subordinate clause has the verb at the end, and the main clause keeps normal word order (verb in 2nd position).

In “sie arbeitet”, how do I know whether sie means “she” or “they”?

In the present tense, “sie” can mean “she” or “they,” but you see the difference in the verb ending:

sie arbeitet = she works (3rd person singular)
sie arbeiten = they work (3rd person plural)
Because the sentence has “arbeitet”, it must be “she.”

Why is it “mit ihren Händen” and not “mit ihre Hände”?

The preposition mit always takes the dative case.
Hände is plural, and the dative plural of die Hände is den Händen.
With a possessive pronoun, den is replaced by ihren (same dative-plural ending -en):

mit den Händen → mit ihren Händen (with her/their hands).
So “mit ihre Hände” is wrong because “ihre” is not the correct dative plural form.

Why exactly is the ending -en used in “ihren Händen”?

The pattern is: mit + dative plural.

Article: die Hände → mit den Händen (dative plural, article gets -en).
With possessive: instead of den, you use ihr-, but keep the -en ending: ihren Händen.
So “ihren” marks dative plural; Hände also gets an extra -n in the dative plural → Händen.
Both the pronoun (ihren) and the noun (Händen) show that this is dative plural.

Does “ihren” here mean her or their hands? How do I know?

In isolation, “ihren Händen” could mean “her hands” or “their hands”; ihr- is ambiguous in German.
Context (who is being talked about) usually tells you which is meant.
Grammatically, it matches a feminine singular “sie” or a plural “sie” for possession.
In a typical context with “sie arbeitet” (= she works), ihren Händen would naturally be understood as “her hands.”

Why is Finger used with meine but without any other article?

In German, possessive pronouns (mein, dein, sein, ihr, etc.) replace an article.
So you don’t say “die meine Finger”, you just say “meine Finger”.
Here “meine” acts like an article and shows that the fingers belong to me and that the noun is plural.

Why is it “mit ihren Händen” and not something like a reflexive construction (sich)?

The verb arbeiten is not reflexive in standard German; you don’t say “sie arbeitet sich”.
The phrase “mit ihren Händen” is simply a prepositional phrase telling you how/with what she works.
So you just say: sie arbeitet (mit ihren Händen) = she works (with her hands), no reflexive pronoun needed.

Is there any special reason to say “mit ihren Händen” instead of just “mit den Händen”?

mit ihren Händen = specifically her hands (or their hands).
mit den Händen = the hands (more general: with hands, with one’s hands).
If you want to emphasize whose hands they are, you use the possessive ihren.
Both are grammatically correct; choice depends on how specific you want to be.

Why are Finger and Händen capitalized?

In German, all nouns are capitalized, regardless of position in the sentence.
Finger and Hände/Händen are nouns, so they must start with a capital letter.
This is a general spelling rule in German, not something special about these particular words.