Learn Hardcore Japanese: kare ha ukkari mezamasi wo kesite, asa ni okirarenakatta. - He carelessly turned off the alarm and couldn’t wake up in the morning.

は marks the topic. It frames what you’re going to talk about: “As for him, …”. It doesn’t add focus; it just introduces the subject of discussion. If you used 彼が, you would be emphasizing that it was specifically him (as opposed to someone else) who did it or couldn’t wake up.

What exactly is うっかり, and how is it used?

うっかり is an adverb meaning “carelessly,” “inadvertently,” or “by mistake.” It typically sits right before the action it modifies:

うっかり目覚ましを消して = inadvertently turned off the alarm.

Variants:

うっかりして目覚ましを消して: adds a sense of “being careless” as a state, then doing the action.
つい vs うっかり: つい often implies doing something unintentionally by impulse/without thinking; うっかり highlights negligence/oversight.

Does 目覚まし mean “alarm clock,” or do I need 目覚まし時計?

Both are fine.

目覚まし is common in casual speech and can mean “alarm (clock).”
目覚まし時計 explicitly says “alarm clock.” People also say アラーム (especially for phones).

Is 消す the best verb for alarms, or should it be 止める or 切る?

All appear in real usage, but nuance differs:

目覚ましを止める: “stop the (ringing) alarm.” Very natural when it’s sounding.
目覚まし/アラームを消す: “turn off/clear the alarm.” Also common; broader “turn off” sense (like lights, devices).
アラームを切る: “cut off/turn off (power or function).” If the alarm was ringing and you stopped it, 止める feels most precise; 消す is still widely used and acceptable.

What does the 〜て form in 消して do here?

The て-form links clauses. It can simply sequence actions (“did X and then Y”) or imply cause/result. Here it reads naturally as cause/result:

“He inadvertently turned off the alarm, and as a result couldn’t wake up.” You could also express the cause explicitly:
うっかり目覚ましを消したので、朝起きられなかった。 Adding しまって strengthens the “accidental/unfortunate” nuance:
うっかり目覚ましを消してしまって、朝起きられなかった。

Why is there a comma after 消して?

The comma 、 is optional and used for readability, especially when the first clause feels like a cause and the second is the result. You can write it without the comma too.

Should it be 朝に or just 朝? What about 朝は?

朝起きられなかった (no に) is very natural; words like 朝/昼/夜 often function adverbially without に.
朝に起きられなかった is acceptable, sometimes a bit more formal or when you want to emphasize the time frame.
朝は起きられなかった uses は to contrast: “In the morning (as for the morning), he couldn’t get up (but at other times maybe he could).”
朝には can mean “by morning” depending on context.

What exactly does 起きられなかった mean, and how is it formed?

It’s the potential past negative of 起きる (to get up).

起きる → potential 起きられる (can get up)
Negative: 起きられない
Past negative: 起きられなかった = “could not get up/wake up” Note: 〜られる can also be passive/honorific in other contexts, but here it’s clearly potential.

What’s the difference between 起きられなかった and 起きなかった?

起きられなかった: “couldn’t get up” (inability, often due to a reason like alarm off, exhaustion, etc.).
起きなかった: “didn’t get up” (a plain statement of non-occurrence, not necessarily implying inability).

Is the colloquial 起きれなかった acceptable?

That’s ら抜き言葉 (dropping ら in the potential). It’s common in casual speech (you’ll hear 食べれる, 見れる, 起きれる), but in formal writing or careful speech, stick with the standard 起きられなかった.

Do I need to include 彼は, or can I omit it?

You can omit it if the subject is clear from context. Japanese often drops pronouns:

うっかり目覚ましを消して、朝起きられなかった。 Use 彼は if you need to introduce or emphasize the topic.

Are spaces normal in Japanese writing?

No. Japanese normally does not use spaces between words. The spaced version is often used in teaching materials. A natural write-up would be:

彼はうっかり目覚ましを消して、朝に起きられなかった。 (or with the stylistic tweaks discussed above)

How do I make this more polite?

Keep the style consistent and put the main clause in polite form:

彼はうっかり目覚ましを止めてしまい、朝起きられませんでした。
彼はうっかり目覚ましを消して、朝は起きられませんでした。 Using 〜てしまい adds an “unfortunately/accidentally” feel that matches うっかり well.

Where can I place うっかり?

Most natural placements are right before or near the verb it modifies:

うっかり目覚ましを消して…
目覚ましをうっかり消して… (also okay, slightly different emphasis)
うっかりして目覚ましを消して… Avoid putting うっかり at the very end; it should sit close to the action.

Can I say 目が覚めなかった instead? How does that differ from 起きられなかった?

目が覚める/目覚める: to wake up (open your eyes, stop sleeping).
起きる: to get up (physically leave bed/be up). So:
目が覚めなかった: “didn’t wake up” (stayed asleep).
起きられなかった: “couldn’t get up” (maybe he woke up but still couldn’t get out of bed, or simply couldn’t wake/get up). Both can fit oversleeping contexts, but they emphasize different stages.

How do you pronounce the key words?

彼: かれ (kare)
うっかり: うっかり (ukkari; note the small っ)
目覚まし: めざまし (mezamashi)
消して: けして (keshite)
朝: あさ (asa)
起きられなかった: おきられなかった (okirarenakatta)