[masaegu]

Originally Posted by OzukakiBurasuki

May I go out tonight?

「今晩私は出かけてもいいですか。」 (今晩 is fine where it is, right? [essentially like using 今日、?)

Correct. Natve speakers would not use the pronoun in this sentence but you are probably required to do so in school?

You must not watch this movie alone. You are 16 years old!
「一人でこの映画を見てはいけません。十六才ですよ。 」 (Could I add ～から because an explanation is given? Is the ～よ okay? o_o)

Correct. You could say 「～～ですから」. The 「よ」 is okay.

I am allowed to work part-time in Japan.
「私は日本でアルバイトしてもいいです。」 (Is this correct? I don't want to get hit again for loving で D

Correct. Love 「で」 all you want here because 「アルバイトする」 is an action verb. Had the sentence been "I am allowed to stay (or live) in Japan.", then the particle would have had to be 「に」.　

I am not allowed to go outside.
「外に行ってはいけません。」 (does this sound awkward? Should I use うちの外 to be more specific?)

Awkward but grammatical. Either 「外」 or 「うちの外」 is okay. The awkwardness is in the verb choice. Native speakers would prefer to use 「出る」 over　「行く」.

I will go to the library and return the book tomorrow.
「私は明日図書館行って、本を返します。」 (does this sound alright?)

No comma needed. Use a particle after the 「図書館」 unless you are attempting the casual speech. But then, no pronoun would be used in casual speech. So there is this imbalance in your sentence. Native ears will rarely, if ever, miss this type of imbalances. Particularly mine!

Mary and Takeshi met and talked for about an hour.
メアリさんとたけしさんは会って、一時間ぐらい話しま した。」 (did I place 一時間ぐらい correctly, or does it matter which verb it has to be placed in front of?)

Correct except for the comma. The 「一時間ぐらい」 can be placed in front of either verb in this case.

My friend went to China and did not return.
私の友達は中国に行って、帰りませんでした。」 (would it be okay to drop 私の?)

Grammatical but not too natural-sounding. We would use 「帰ってこなかった」.　In this sentence, you have more of a reason　to use a comma than in the couple of previous sentences. That is the unspoken, rather long time lapse between the two actions.

I am not free today. (It's) because I have a test tomorrow.
「今日私は暇ではありません。明日テストがありますか ら。」 (does this sound alright?)

Correct.

The test was not difficult. (That was) because I had studied a lot.
「テストは難しい区ありません。たくさん勉強しました から。」 (was my use of たくさん correct or incorrect?)

「難しい区」　＞　「難しく」.
Your use of 「たくさん」 is okay-ish but not recommendable for an adult speaker. Use 「よく」. Better yet, use 「一生懸命（いっしょうけんめい）」 if you have learned it.

Let's go out tonight. (It's) because tomorrow is a holiday.
「今晩は出かけましょう。明日休みがありますから。」 (would I need to make 今晩 the topic, or can I drop は?)

Yes, you need to because "tonight" is special because of what "tomorrow" is, which is a holiday. Change the second half to 「明日は（お）休みですから」.

I helped my mother. (It's) because she was busy.
「私はお母さんを手伝いました。お母さんは忙しかった ですから。」 (Is it weird to say お母さん twice, or is my English just making me want to use 彼女 instead?)

If the speaker is over around 11-12, use 「母」 instead of 「お母さん」. You can use 「母」 twice. Almost no native speakers would use 「彼女」 here. Those who do are too heavily influenced by a foreign language.

「私はお母さんを手伝いました」 sounds like it was translated from a foreign language, which is what happened. I recommend 「母の手伝いをしました」.

IMPORTANT: Use 「が」 for the second half.

I will not drink coffee. (It's) because I drank coffee in the morning.
「私はコーヒーと飲みません。朝コーヒーを飲みました から。」 (this sounds weird to me)

Why 「と」? The rest is good.

I will not meet you tonight at the tea house/cafe. (It's because) I have to study for a test tomorrow.
「今晩私は喫茶店であってはいけません。」 (does this sound fine?)
「明日テストを勉強しますから。」 (did I word the second sentence right?)

Both incorrect. First one sounds as if one was talking about a forbidden love of a sort. Second is plain weird.

「今晩喫茶店には行けません。」or 「今晩喫茶店で会うことはできません。」
「明日のテストの勉強をしますから。」