Hi
Please see below:
1- Do not fret[color=red], she will get here in time.
2- Do not fret[color=red]; she will get here in time.
3- Do not fret[color=red]. She will get here in time.
4- Do not fret[color=red]- she will get here in time.
Are all the sentences correct? If yes, where are the rules of punctuation leading us?
Tom
Hi, Tom
Do punctuation marks trouble you today? 
Sentences 2 and 4 are probably incorrect.
In addition to 1 and 3.
Do not fret! She will get here in time.
Just my thoughts.
P.S. English Grammar books provide good explanation of usage the punctuation marks.
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I’m going to wait for the punctuation guru (MM). 8)
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1- Do not fret, she will get here in time. – wrong; this is a comma splice error (a comma but no conjunction between independent clauses.
2- Do not fret; she will get here in time. – fine
3- Do not fret. She will get here in time. – Punctuation is fine; one point off for choppy style: a series of short simple sentences that would be better more closely associated.
4- Do not fret– she will get here in time. – (Notice that the convention is two hyphens for an m-dash) This is fine in informal (and these days, increasingly formal) writing.
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Mister Micawber, Amy is absolutely right: You are our Punctuation Guru![YSaerTTEW443543]
TOEIC listening, photographs: A man running under his umbrella[YSaerTTEW443543]
Mister Micawber, from your explanation I understand that
Do not fret,( conj) she will get here in time is correct, right?
I consider this sentence as a compound one. I’ve read that independent sentences are usually joined by means of a comma and a coordinating conjunction.
But in one of my grammar books I found such an example:
The train fell softly, the house was quiet. (joined asyndetically) Personally, I’d used semicolon instead.
Give me some feedback, please.
Do you agree with me that the 2nd sentence given by Tom is the best choice?
Semicolon binds two independent clauses more closely.
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Yes, plenty of these exist, and it is often an effective literary technique. For students, however-- and I don’t know how to remind people continuously that I am speaking for the benefit of learners at this site-- the comma splice is a common and often tested grammar ‘mistake’.
Yes, the semicolon is the best choice here (even though I myself am using m-dashes more and more).
As for being a guru-- just wait till I mess up on the quotation marks!
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Thanks a lot, Mister Micawber!