Hello. I will be very grateful for your help.

1)Does the following sentence sound natural English: "someone fired a mortar at a building just 100 meters away from my house"?
2)Which preposition is correct in the phrase: "the bank account is opened at/ with OTP Bank"?
Thank you very much.

All are correct. We can use either at or with in the second sentence.

1)Does the following sentence sound natural English: "someone fired a mortar at a building just 100 meters away from my house"?

OK, if you capitalize the S on the first word.

2)Which preposition is correct in the phrase: "the bank account is opened at/ with OTP Bank"?
Neither, unless you change the tense of that verb. Maybe "... was opened at OTP Bank." That is, if you mean it has already been opened there.

Hello! I'd be happy to help you with your questions.

1) The sentence "someone fired a mortar at a building just 100 meters away from my house" sounds natural in English. It is clear and grammatically correct.

2) The correct preposition to use in the phrase "the bank account is opened at/with OTP Bank" is "with." You would say "the bank account is opened with OTP Bank." This indicates that OTP Bank is the institution where the account is being opened.

If you would like to check the naturalness of sentences in the future, you can use online resources like grammar and writing forums, native speaker language exchange platforms, or simply ask friends or native English speakers for their opinions. Additionally, to determine the correct preposition to use in phrases, it's helpful to refer to grammar resources or consult style guides.