Hi and welcome back!
I would just make two suggestions:
漫画喫茶に行くのは、僕の姉 の方です。
If you say ‘漫画喫茶を行く’ it sounds as if you went through the manga café. Here you can find more information on the differences between を行く and に行く.
As for the addition of の方, please take a look at this grammar point.
Finally, it wouldn’t be possible to say 行きますのは as the polite form ます goes at the end of the sentence. The only exceptions allowed are presented here.
HTH!
Hi Pablo,
The の方 part is unnecessary. The rest of what you said is good though 
.
Minor nitpick, but you’re treating the verb 「別れる」like it’s a Godan (う) Verb here, where it’s actually an Ichidan (る) Verb. The correct conjugation for polite past would be 別れていました instead of 別れっていました。
Here’s a useful verb conjugation thread for you to look at if you’re interested.
I also believe this is an improper use of the polite form of the verb, and should instead be
「…別れていた分かりません。」
Hope this helps!
２０１９年１２月に日本語能力試験のN４を合格したが、コロナウイルスのせいで、証明書を受け取りませんでした。
I passed the JLPT N4 in December 2019, but because of Covid, I never got my certificate.
(I requested a new one yesterday)
Thank you for the correction! So it should be more like
「なぜ彼女が私と別れていた分かりません」？
僕の仕事は難しすぎたから止めて海の下ヘ二万リグズ行った。
Work was too hard so I quit and went 20,000 leagues under the sea.
I am open to corrections.
It should be:
「なぜ彼女が私と別れたか分かりません。」
You want to use the particle と here, to mean “with” me (not “to” me or “at” me like the particle に would imply). You’re also missing the か particle at the end of 別れる, which is functioning as the question marker in this case. Lastly, you want 別れる to be in the past tense (別れた) since you don’t continuously break up with someone. It just happens once and it’s done.
HTH!
I was attempting to use this grammar point:
You are dead and remain in the state of being dead/she broke up with me and we remain broken up. Does it work differently because the act of breaking up and remaining separated are different verbs, but しぬ applies both ways?
In Japanese, are “I don’t know why she broke up with me.” and “why did she break up with me?” both marked with か because in both cases I don’t know the answer?
As far as I know, because I’m not 100% sure in this case, the action of breaking up (別れる) is not continuous. But if you were to use the verb 離れる (はなれる), then you can use the ている form because being separated is continuous. Being dead is also continuous.
I believe that the definition of “To separate” in Jisho is more like the action of “To break up”, since we’re talking about a couple in this case. This is why it’s important to see the word in action because context is super important in order to really understand the actual meaning of words. A lot of words sound similar, but their meaning might be different than what you’re thinking.
It has to do with there being a question inside a phrase. “Why she broke up with me? I don’t know.” That’s literally what you’re saying in Japanese. And since questions are typically marked by か, then you use it to mark it as such. Does that make sense?
Thank you, that does make sense. What a trip.
勝ちに近いと思っているところだったが「ロン」と聞いた。
Just as I thought I was close to winning, someone said “Ron!”.
六万九千四百二十は本当に可笑しい番号です。
69420 is a really funny number.
英語を話す国に、計算機科学の学生達が初めて作ったアプリは、 たいてい、モニターに"Hello World"が書けるアプリです。
In English-speaking countries, the first program computer science students write is usually one that prints “Hello World” to the screen.
ほら！先生が聞かないふりをしている内に先生に関係がある噂を教えてね。
Hey! While the teacher pretends not listen, tell me a rumour related to him, OK?
妻にバスケットボールサイズなダイアモンドを買ってあげました。私の財布は現金がないけど心はお金持ちです。
I bought my wife a basketball sized diamond. My wallet is broke but my heart is rich.
I am open to corrections.
Hi!
I think it’s perfect, your wife and your heart might think so too. Nice use of は in its contrastative role.
The only thing I would comment is that both バスケットボール and サイズ are names (名詞), so it should be バスケットボールサイズのダイアモンド or バスケットボールのサイズのダイアモンド.
So if translated literally, would that mean the concept of “basketball sized” is taking ownership of the diamond?
Edit: fixed a typo
The particle の has many functions. It’s not just a possessive particle, so don’t think of it as only having that function.
In this case, の is simply connecting nouns, as Pablo mentioned. So “A basketball-sized Diamond” becomes バスケットボールサイズのダイアモンド. It’s not that “Basketball-sized” owns “Diamond’. That’s not it 
.
ペンギンを見るために南極にヘリコプターで行った。それからイグルー押し入て一つをペットとして家に帰りました。今彼はフロゼン野菜とフリーザーに住んでいます。
I took a helicopter to Antarctica to see the penguins, then broke into an igloo, took one as a pet and returned home. He lives in the freezer with the frozen vegetables now.
There are a couple of mistakes here:
For one, you want to be consistent with the politeness level. For example, the first sentence is in plain form, while the others are in polite form.
The second sentence is using a general counter for Penguins, which is the incorrect counter. In this case, you’d use the 羽 counter, which is used for small birds and rabbits (I know, lol. Blame the monks for that one). The verb usage is strange too, so I replaced them.
Lastly, in the third sentence, you’re using 彼 as the pronoun, but I think Japanese people either use (name of pet) + ちゃん, or the sound the animal makes + ちゃん. When the name is not known, I believe they simply say (animal) + さん. I’m not 100% on this, since it might sound a little childish, but that’s the pattern I’ve seen. Perhaps you can just stick to ペンギン by itself.
Anyway, the end result would be something like the following:
ペンギンを見るために南極にヘリコプターで行きました。それからイグルに入ってペットとして一羽を取って家に帰りました。今ペンギンはフロゼンの野菜とフリーザーに住んでいます。
HTH!
