Question

the lady listened carefully to my complaint but I doubt she will do anything about it.​

Answer 1

"But I doubt" is the grammatical error in this sentence.

• Grammatical error is a terminology which used in rigid grammar
• Learning proper grammar is significant as it is the language that makes it possible for us to efficiently talk about language.  
• Grammar is imperative since it is the language that makes it conceivable for us to talk about language.  

Hope it helped...