Learn Hardcore Japanese: asita made ni owaru ka douka, watasi ha mada wakarimasen. - I don't know yet whether it will be finished by tomorrow.

bold まで marks an endpoint of a continuing state: “until.”
- 昼まで勉強する = study until noon (the activity continues up to noon).
bold までに marks a deadline by which an event happens: “by.”
- 明日までに終わる = finish by tomorrow (at or before the end of tomorrow). With punctual verbs like bold 終わる, bold までに is the natural choice.

Why is the verb inside the clause in plain form (bold 終わる) and not polite (bold 終わります)?

In embedded questions (〜bold かどうか / 〜bold か), the verb stays in plain form. Politeness is expressed at the end of the sentence in the main predicate (here bold 分かりません). So bold 終わるかどうか分かりません is correct; bold 終わりますかどうか is not.

What exactly does bold かどうか mean? Can I drop bold どうか?

bold かどうか means “whether or not.” You can often drop bold どうか with verbs like bold 分かる: 明日までに終わるか分かりません ≈ “I don’t know whether it will finish by tomorrow.” Keep bold どうか when:

You want to emphasize both possibilities, or
You attach topic/case particles to the clause: 明日までに終わるかどうかは… / …かどうかを心配している. Saying …bold かは… is generally unnatural; use …bold かどうかは….

Why is bold 私は after the clause? Could I put it first?

Japanese allows fronting the clause to set the topic/frame: “As for whether it will be done by tomorrow, as for me, I still don’t know.” All are fine:

私は明日までに終わるかどうか、まだ分かりません。
明日までに終わるかどうか、私はまだ分かりません。
明日までに終わるかどうかは、まだ分かりません (clause itself as topic).

Do I need to say bold 私? What about bold 私には?

You can drop bold 私 when it’s obvious: 明日までに終わるかどうか、まだ分かりません. Using bold 私には adds “for me / from my perspective,” slightly emphasizing the experiencer: 明日までに終わるかどうか、私にはまだ分かりません. Avoid 私が分かりません (that means “I am not understood”).

Who is the subject of bold 終わる here?

It’s omitted. It could be “the task/work” (intransitive: “the task finishes”), or contextually “I will finish it.” Japanese commonly leaves subjects implicit when clear. If you want to stress the doer’s ability, switch to a transitive verb: 明日までに終えられるかどうか (“whether I can finish it by tomorrow”).

Why bold 分かりません and not bold 知りません?

bold 分かる = to understand / be clear; used for certainty or (un)clarity: まだ分かりません = “I’m not sure yet.”
bold 知る = to know (as information) / to come to know. まだ知りません means “I don’t yet know (haven’t learned the info).” Here bold 分からない is the default; bold まだ知りません is possible but leans toward “the information isn’t available to me yet.”

Where does bold が go with bold 分かる?

The thing understood normally takes bold が: Xが分かる. Fully explicit: 私は明日までに終わるかどうかがまだ分かりません. In your sentence, the clause is fronted, so the bold が is simply omitted after fronting. You can also topicalize it: 明日までに終わるかどうかは、まだ分かりません.

Does bold 終わる here mean future “will finish”? There’s no future tense.

Yes. Japanese non-past (dictionary form) covers present and future. With the time expression bold 明日までに, bold 終わる is understood as “will finish (by tomorrow).”

Could I say bold 終わっているかどうか or bold 終えられるかどうか instead? What’s the nuance?

bold 終わっているかどうか: focuses on the resultant state “be finished” at that time (“whether it will already be finished by tomorrow”).
bold 終えられるかどうか: potential of transitive 終える (“whether I can finish it by tomorrow,” emphasizing ability/feasibility). Choose based on whether you care about the state, the fact of occurrence, or your ability.

Why not use bold 終われる for “can finish”?

bold 終わる is intransitive (“to come to an end”), and its potential bold 終われる means “can come to an end,” which is rarely what you want. For “I can finish it,” use transitive bold 終える → bold 終えられる, or causative-potential bold 終わらせる → bold 終わらせられる.

What does bold まだ add, and where should it go?

bold まだ + negative means “not yet / still not.” まだ分かりません = “I still don’t know yet.” It typically sits before the verb phrase: 私はまだ分かりません. Contrast with bold もう (“already/no longer”): もう分かりません often means “I no longer understand.”

How would I say this more casually or more formally?

Neutral/polite (your sentence): …まだ分かりません。
Casual: …まだ分からない。 or …まだ分かんない。
Very polite/formal: …まだ分かりかねます (business; “I am unable to say yet”) or …現時点ではまだ分かりません.

Is the comma bold 、 necessary? Do I use a question mark?

The comma is optional but helps readability when fronting the clause. No question mark is used because the sentence isn’t a direct question; bold か/かどうか just marks an embedded question. So both are fine: 明日までに終わるかどうか、私はまだ分かりません。 / 明日までに終わるかどうか私はまだ分かりません。

Why are there spaces between words in the example?

They’re just pedagogical spacing. Standard Japanese doesn’t insert spaces between words: 明日までに終わるかどうか、私はまだ分かりません。

Should there be bold こと after the clause (…かどうかこと)?

No. bold か and bold かどうか already nominalize the clause, so you don’t add bold こと. You might see …かどうかということ in very formal or explanatory writing, but it’s not necessary here.

Can I use bold かどうか with WH-words like “when/why,” or should I use bold か?

Use bold かどうか only for yes–no questions. With WH-words, use bold か:

いつ終わるか、まだ分かりません。= “I don’t yet know when it will finish.”
終わるかどうか、まだ分かりません。= “I don’t yet know whether it will finish.”