Learn Hardcore Portuguese (Brazil): Depois que o contrato for assinado, a mudança pode começar. - After the contract is signed, the move can begin.

Depois que introduces a clause with a verb (a full action/event): Depois que o contrato for assinado = after the contract is signed.
Depois de is normally followed by a noun or an infinitive:

Depois de assinar o contrato, ... = after signing the contract
Depois do contrato, ... = after the contract (less specific)

What tense/mood is for assinado? Why isn’t it será assinado?

for assinado is future subjunctive of ser (for) + past participle (assinado) in a passive construction.
Portuguese typically uses the subjunctive after time expressions like depois que, quando, assim que when the event is in the future or not yet completed.
So it’s “after the contract is signed (in the future)” → Depois que ... for assinado.
será assinado (future indicative) is grammatically possible in some contexts, but it often sounds more like a firm prediction/statement and is less common after depois que than the subjunctive in everyday Brazilian Portuguese.

How do I know for comes from the verb ser?

It’s an irregular form. The future subjunctive of ser is:

quando eu for
quando você/ele for
quando nós formos
quando vocês/eles forem

So o contrato for assinado literally uses ser in the subjunctive: “that the contract be signed.”

Is o contrato for assinado passive voice? Who is signing?

Yes. ser + participle = passive voice: ser assinado = to be signed.
This sentence doesn’t mention the agent (the signer), which is very common when it’s not important or is obvious from context. If you wanted to include it, you could add por:

Depois que o contrato for assinado pela empresa, ...

Why is the participle assinado and not assinada?

In the passive voice, the past participle agrees with the subject of the passive clause. Here, the subject is o contrato (masculine singular), so the participle is assinado.
If it were a proposta, you’d get: Depois que a proposta for assinada...

Could I say Depois que o contrato estiver assinado instead?

Yes, and it’s common. estar + participle focuses more on the resulting state (“once it is signed / once it’s signed and done”):

Depois que o contrato estiver assinado, ...
ser + participle focuses more on the event/process (“after it gets signed”).

Why is there a comma after assinado?

Because the sentence starts with an introductory dependent clause (Depois que...). In Portuguese, it’s standard to separate that introductory clause from the main clause with a comma:

Depois que X, Y.

What does a mudança mean here—“change” or “move”?

mudança can mean both change and a move/relocation. In a context with contracts, it often means a relocation (e.g., moving house or moving offices), but the sentence alone allows either meaning. The rest of the text usually makes it clear.

What exactly does pode convey in a mudança pode começar?

pode (from poder) expresses possibility/permission/feasibility depending on context:

“the move/change can start” (it becomes possible)
“the move/change may start” (it becomes allowed)
It’s less absolute than vai começar (“will start”).

Why is it pode começar (infinitive) and not a conjugated verb?

After modal verbs like poder, the next verb usually stays in the infinitive:

pode começar = “can start”
You don’t conjugate começar there because pode already carries the tense/person.

Could I replace Depois que with Quando or Assim que?

Often, yes, with slightly different nuance:

Quando o contrato for assinado... = “when the contract is signed...” (neutral timing)
Assim que o contrato for assinado... = “as soon as the contract is signed...” (immediacy)
Depois que o contrato for assinado... = “after the contract is signed...” (sequence emphasized)