Learn Hardcore Japanese: kare ha tyokin no tame ni, hiruma ha okane wo tukaisuginai you ni suru. - He tries not to spend too much money during the day in order to save.

N + の + ために = "for the sake of N / for N." So 貯金のために = "for savings."
V(dict.) + ために = "in order to V." So 貯金するために = "in order to save (money)."
Both are natural here. With the noun, the purpose is framed as a goal; with する, it sounds a touch more like an intended action. Meaning difference is minimal in this sentence.

Does ために mean "for" or "because of" here? How can I tell?

It means purpose ("for/in order to"). Rule of thumb:
- If the main clause is a volitional/controllable action by the subject → purpose.
- If the main clause is a result or uncontrollable state → cause.
Here the main clause is 〜ようにする (a deliberate effort), so it's purpose.

Why is 昼間 marked with は? Could I use 昼間に instead?

昼間は sets "daytime" as the topic and is often contrastive: "as for the daytime (as opposed to other times)..."
昼間に simply marks the time when the action happens: "during the day."
Both are fine. は sounds like a general policy that contrasts with other times; に is a neutral time adverbial.

Is it okay to have two は in the same sentence (彼は … 昼間は …)?

Yes. 彼は is the overall topic ("he"), and 昼間は is a subtopic ("as for daytime"). Japanese can stack topics like this when they refer to different things.

What does ようにする add? Why not just say 昼間はお金を使いすぎない?

V-ないようにする = "make an effort to avoid V / try not to V / ensure that you don't V."
Without ようにする, 使いすぎない reads as a simple statement of fact or habitual truth. With ようにする, it emphasizes intention and self-control.

Difference between ようにする and ようにしている?

ようにする: a decision/effort from now on or in specific situations ("I'll try to...").
ようにしている: an established habit or ongoing practice ("I make it a habit to...").
In your sentence, if it's a standing habit, している is very natural.

Difference between ようにする and ことにする?

ことにする = "decide to do (once/firmly)." With a negative: 〜ないことにする = "decide not to."
ようにする = "try to / make sure to" (focus on effort and ongoing behavior).
Example:
- 昼間はお金を使いすぎないことにした。= I decided not to overspend during the day.
- 昼間はお金を使いすぎないようにしている。= I make it a habit not to overspend during the day.

How is 使いすぎない formed? Why 使い and not 使わ?

Base verb: 使う.
ます-stem: 使い. Attach すぎる ("to overdo"): 使いすぎる.
Negate the auxiliary すぎる: 使いすぎない ("not overspend"). The negative applies to すぎる, so the stem is 使い, not 使わ.

Can I write 使い過ぎない with the kanji 過? Is that normal?

Yes: 使い過ぎない is common, and so is all-kana 使いすぎない. Both are fine. Avoid the old 遣い spelling in modern writing.

Why is お金 marked with を? Could it be お金は instead?

お金を marks the direct object of 使いすぎない.
お金は is also possible: お金は使いすぎないようにする. Using は topicalizes "money" and can imply contrast ("as for money, I don't overspend").

Could I say 昼間には?

Yes, 昼間には combines the time marker に with the topic は, adding emphasis/contrast: "at least in the daytime..." It’s a bit stronger than just 昼間は or 昼間に.

Is 彼 necessary? Would natives say it?

Pronouns like 彼 are often omitted when the subject is clear from context. A very natural version is simply: 貯金のために、昼間はお金を使いすぎないようにする／している。
In narratives or when introducing a male person, 彼は is fine.

Can I move 貯金のために or drop the comma?

Word order is flexible:
- 貯金のために昼間は… (no comma) is fine.
- 昼間は…、貯金のためだ。 (split into two sentences) is also natural.
Avoid ending the sentence with …, 貯金のために。 Keep it as a modifier to the verb or turn it into ためだ if you place it after the main clause.

How do I make this polite?

Use polite endings:
- Habit: （彼は）貯金のために、昼間はお金を使いすぎないようにしています。
- Resolution/decision: …使いすぎないようにします。

Why are there spaces between the words here, and how do you read the sentence?

Standard Japanese doesn’t use spaces; they were added to show word boundaries for learners.
Readings: 彼(かれ) は貯金(ちょきん) のために、昼間(ひるま) はお金(おかね) を使(つか)い過(す)ぎないようにする。