Formal:
* Ambiguous: This indicates that the sentence can be interpreted in multiple ways.
* Unclear: This is a general term for a sentence that is difficult to understand.
* Confusing: Similar to "unclear", but implies more difficulty in understanding.
* Vague: This implies that the sentence lacks specific details or information.
* Obscure: This means that the sentence is difficult to understand because it uses unfamiliar language or concepts.
* Lacking clarity: This is a more formal way of saying "unclear".
Informal:
* I don't get it.
* What does that mean?
* Can you rephrase that?
* That's confusing.
* I'm lost.
* That doesn't make sense.
Specific to the issue:
* The meaning is unclear.
* The subject is not clear.
* The verb is ambiguous.
* The sentence structure is confusing.
* There is a lack of context.
You can also use phrases like:
* Could you please explain that in a different way?
* I need more information to understand this.
* Can you give me an example?
The best way to say that a sentence is not clear depends on the situation and your relationship with the person you are talking to.