Had you better go at once.
(Can we use this sentence?)
It's a question and needs a question mark at the end.
It's o.k.
1. Had you better go at once?
2. Hadn't you better go at once?
(Thank you. Are both OK?)
Both are OK.
Yes, the sentence "Had you better go at once" is grammatically correct. It is a conditional sentence that suggests a strong recommendation or advice. However, it is worth noting that this sentence may sound formal or old-fashioned in modern English. A more commonly used variation is "You should go at once" or "You'd better go at once."
To form this type of conditional sentence, you can use the construction "had + subject + better + base verb." In this case, "had" is the past form of "have" used for creating a strong advice or warning.
As an AI language model, my purpose is to provide information and help with questions. If you have any other questions or need further clarification, feel free to ask.