Hello. I will really appreciate some help.

Do you think it's possible to say: "She regrets not buying the house" or is it "she regrets not having bought the house", maybe both?
And one more question, please. Would you use articles in the context: "in this composition he used a number of instruments, a (piano), a (?)saxophone,an (?) acoustic and bass guitar, etc"
Thank you very much for help.

Both are correct -- although the first is more common.

You could use articles -- but they're not necessary -- in your second example.

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

Regarding your first question, both "She regrets not buying the house" and "She regrets not having bought the house" are grammatically correct. The choice between them depends on the context and the intended meaning.

- "She regrets not buying the house": This implies that she did not buy the house and now feels regret about that decision. The focus is on the action of not buying the house.

- "She regrets not having bought the house": This suggests that she had the opportunity to buy the house in the past, but she didn't take it, and now she feels regret about that missed opportunity. The emphasis is on the completed action of not having bought the house.

As for your second question, in the context you provided, articles are used with some of the instruments. Here's the breakdown:

- "He used a number of instruments": This is correct, as "a number of" implies an unspecified quantity.

- "a (piano)": This is also correct, as the word "piano" starts with a consonant sound. The indefinite article "a" is used before consonant sounds.

- "a (?)saxophone": This would depend on the actual saxophone being referred to. If it starts with a consonant sound (e.g., a tenor saxophone), then you would use "a." If it starts with a vowel sound (e.g., an alto saxophone), then you would use "an."

- "an (?)acoustic and bass guitar": Again, this depends on the actual guitars being referred to. If they start with a consonant sound (e.g., an acoustic guitar, a bass guitar), you would use "a." If they start with a vowel sound (e.g., an electric guitar), you would use "an."

I hope this helps! If you have any further questions, feel free to ask.