In Engpsh, we refer to "key sentences" as "key clauses" or "key phrases". These are sentences or phrases that convey important information or ideas within a larger piece of writing or speech.
For example, you might say:
"The key clause in the contract is the one that outpnes the terms of payment."
"The key phrase in the speech was 'we must stand together'."
Alternatively, you can simply refer to these sentences or phrases as "important sentences" or "important clauses" or "important phrases".
I hope this helps! Let me know if you have any other questions.