Learn Hardcore Japanese: kutusita ga yogoreta kara, sentakuki de araimasu. - My socks got dirty, so I will wash them in the washing machine.

In 靴下が汚れたから, が marks the subject of the intransitive verb 汚れる (“to get dirty”). You’re saying “the socks got dirty,” so the socks are the grammatical subject.

は would mark a topic and shift the nuance (“As for the socks…”).
を cannot mark the subject of an intransitive verb; it marks direct objects of transitive verbs.

Why is the verb 汚れた (intransitive) used instead of 汚した (transitive)?

汚れる is intransitive (“to become dirty”); it describes something getting dirty on its own (or by external dirt). 汚す is transitive (“to make something dirty”), requiring an agent. Because you’re stating that the socks ended up dirty (not that someone dirtied them), you use the intransitive 汚れた.

What function does から have in 靴下が汚れたから and why is it placed after the plain form verb?

から here means because or since, indicating reason. It always attaches to the plain (dictionary or plain past) form of verbs or adjectives before the main clause.
Structure:
[Reason clause in plain form] + から, + [Result/main clause].

In 洗濯機で洗います, what does the particle で indicate after 洗濯機?

で marks the means/instrument or location of an action. Here it means “using a washing machine.” So 洗濯機で洗います = “(I) wash (them) with/using the washing machine.”

Why is the direct object marker を omitted before 洗います? Shouldn’t it be 靴下を洗います?

You can say 靴下を洗濯機で洗います, but Japanese often omits を when the object is clear from context. Because the socks are already mentioned, dropping を靴下 makes the sentence more natural and less repetitive.

Is it acceptable to mix the plain form 汚れた with the polite form 洗います in the same sentence?

Yes. It’s common to use plain forms in subordinate clauses (like reasons or conditions) and switch to polite forms in the main clause. Learners often do this without any problem.

What is the implied subject of 洗います, since there’s no pronoun?

Japanese frequently omits pronouns when the subject is obvious. Here the implied subject is I (or “we,” if context implies multiple people), so it means “I will wash (them).”

How does から differ from ので when giving a reason?

Both mean “because,” but:

から is more direct and subjective (“I think/say X, so Y”). It’s common in everyday speech and pairing with plain or polite forms.
ので is slightly more formal, softer, and implies an objective reason. It often appears in polite or written contexts.
Example difference:
靴下が汚れたから洗います (casual/plain–polite mix, direct)
靴下が汚れたので洗います (more polite/formal, softer)