Which one is better to use?
You'll be fine by/until tonight.
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NOT A TEACHER **********
Hello, Mr. McKane.
(1) Of course, I may be completely wrong, but this is how I interpret your
sentences.
(2) A patient and a doctor are talking:
"I feel very sick.
Don't worry, Mr. Smith. Just take this medicine
and I guarantee you that you will feel fine by tonight. Thank you
so much doctor. It's now 1 p.m. I shall look forward to feeling better
when nighttime comes."
(3) A doctor and a patient are talking:
"Mrs. Jones, I want you to start taking this medicine right now.
Oh,
Doctor, I do not really need any medicine. I feel fine. Listen, Mrs.
Jones, if you do not take this medicine, you will feel fine until tonight,
and then you will suddenly become very sick."
Thank you