Dozens of times a day, people post in the form, “No. The reason is that…”
“No” is not a sentence. Sentences have subjects and objects. That one has neither. Therefore, it requires no period. Unless you’re writing ad copy for a steam-powered automobile in 1902, you do not put a period after a single word. It makes you sound like a schoolmarmish scold, which I am fully aware I am being right now.
Try, “No - the reason is that…” or “No, the reason is that…”.
Thank you.
Really? Wow!
Before someone jumps all over me, the mark in the suggested sentence I gave was supposed to be a dash (–), not a hyphen (-).
No!?
With apologies to our English cousins, bollocks.
I submit the case where one answers ‘No.’ to an asked question, then begins an unrelated sentence.
Punctuation exists to describe how words are spoken. Periods for full stops, commas for pauses, exclamation points for emphasis, et cetera. A full-stop after ‘No.’ is thus perfectly acceptable, if one intends a full stop.
What’s the difference? One pixel maybe.
Well, it sort of can be a sentence, because “no” can be used as an interjection and interjections are often punctuated as sentences. Such as: Wow! Dude! Fuck! (Used here in the “Fuck! I just shit myself.” declarative sense, not the “(You) Fuck!” imperative sense). So it’s not a “proper” sentence, but it’s not wrong to punctuate it as one. In diagramming such a “sentence” is plotted on a single straight line.
Well if we’re going to be pendants, that’s not true either. Sentences must have a subject and a verb. And even the subject can be dropped in some cases.
Stop!
Gaudere’s Law. Pedants, isn’t it?
Yes.
Actually, I’ve heard that: Interjections! show excitement or emotion
They’re generally set apart from a sentence by an exclamation point
or by a comma when the feeling’s not as strong.
Maybe.
Couldn’t just leave that one dangling. 
Hallelujah!
Naturally. Just doing my part. 
Darn. That’s the end.
That was how they described it back in my grade school days. For instance, the subject in the sentence, “Run away!!” is an understood ‘You’.
Same deal with ‘No.’ or ‘Sure.’ or a lot of other one-word responses. They’re sentences; their subject and verb are, by implication, those of the sentence they’re responding to.
“Will you take out the trash?” “No.”
means:
“Will you take out the trash?” “No, I won’t take out the trash.”
Our language has evolved in a way that doesn’t require unneccesary repetition.
:smack:
My goodnes, you mean I’ve been inadvertantly teaching my children incorrect grammar all this time?
“Can I stay up late?”
“No.”
“I don’t want green beans. Can I have ice cream instead?”
“No.”
Frequently, “no” all by itself can be very effective. I shouldn’t have to add an explanation.
So there. 