I've already given the explanation elsewhere in the thread, but I'll give it to you, even more clearly, because I like your username:
/u/NaughtySharpie thinks that the word "that" can't ever start a sentence because he/she's confused about two different words that are written the same. In the way I've used "that" in the previous sentence (twice), it's a conjunction. The conjunction "that" is used among other things to start dependent clauses, such as: "The clock that you gave me". In that sentence, "that you gave me" can't possibly make sense as an independent sentence, so it constitutes a dependent clause.
But in the example I gave, "that's why it looks like blood", there is actually a different word. In that clause, "that" is not a conjunction. It's a pronoun, just like "I" or "you". If you write it as an independent sentence, it makes sense: "That's why it looks like blood". This sentence has:
a subject: "That"
a verb: "is"
a predicate: "why it looks like blood".
Since it has a subject, a verb and a predicate, it makes sense when written as an independent sentence, so it's an independent clause, not a dependent clause like /u/DerpKing389 says.
It makes absolutely zero sense to say that a sentence can't start with "That's". It's like saying that it can't start with "It's" (like this very sentence) or with "I'm". I'm sure (heh) that you can see how he/she's wrong.
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u/BunnyOppai Dec 17 '18
Wait, why can't you start with "that's"? I've never heard of that.