*grammar Nazis :v

2 and 3, can't really help as I'd need to be able to have a good understanding of the original Japanese, which I don't. All I can say is that it doesn't make sense within the situation.

4   "something up her sleeve" would normally be associated with a trick (as in magic). So something like " Let's see if Midori has any tricks up her sleeve" would seem to make more sense, though again I can only base this on your script as my knowledge of Japanese is obviously vastly inferior to yours.

Haven't watched it yet, but... After help, no "to".
My English is badest, very!
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Indeed it's not the perfect sub, but people can be content with even worse!

Ty Ram-kun.

My guesses:

1. More often than not, singular common nouns are preceded with a quantifying particle such as 'a'. Particles can be quite confusing to non-native speakers, so hope that helps.

2. The sentence seems grammatically correct to me, and likely more of a nitpick than anything else. "Let us" is almost always contracted to "Let's" in speech (and most anime speech is casual), with the former sounding somewhat more formal. For help...to construction, often the 'to' particle is dropped; leaving it in can sound a little unnatural, but seems grammatically fine.

3. I think the sentence is fine, but just an odd thing to say in English. Perhaps "Don't worry!" or similar is what the parent poster was looking for, though it's a little less accurate. I think this may just depend on how literal you want to be with your translations, for which everyone will have their own preference. Personally, I'd agree with the suggestion, as I don't feel that "don't worry" is different enough to justify the odd English. Still, I'd say it's more a nitpick than anything else.

4. The usage of proverbs can be a little tricky, and care needs to be taken that the correct context is established. With this one, it needs to be a variation of "have up my sleeve" - your sentence lacks some form of "have" or "has", hence somewhat confusing. Something like "Next, Midori will show you something she has up her sleeve" is more correct. As an above poster mentions, the expression is often used in the context of magic tricks, and is something that will be revealed, hence the verb 'show' is more appropriate. Tricks can be taught, what's up one's sleeve is shown (not taught).

Hope that's helpful. English is quite a whacky language to grasp, particularly natural English as there's generally no defined rules (you just have to have enough exposure to it, and even then, that may not be enough). Thanks for all your efforts!