In graduate school, a particular professor excoriated (a friend) for
starting sentences with “Though.” Though, if “although” and “though” are
synonymous – as several web sources seem to suggest – why is it okay to start a
sentence with one and not the other? Or was Professor Particular simply acting
as a Grammar Snob?
There's nothing grammatically wrong with starting a sentence with "though" or "although." But this practice can set up a very bad sentence. Long introductory clauses and phrases can suck the life out of a sentence like nobody's business. I suspect that's what the professor was talking about.
There are two ways such introductory matter can hurt a sentence: 1. by demoting the main clause/point of the sentence to a much lower position or, worse, 2. by cramming important information in as mere introductory matter.
"Though he had killed everyone in the house with a rusty ladle and served their organ meat to the dogs, he wasn't tired."
What's the main clause in this sentence? It's "he wasn't" (tired). All the interesting stuff is crammed in before a comma in a portion that reads like it's squeezed into one deep breath. (Read the sentence aloud or hear it in your mind and notice implied exhalation at the comma. Notice the hurried tone before you get to the comma. )
This is why "though" and "although" can be a bad way to start a sentence. They can also be a great way to start a sentence if the writer knows what she's doing. My guess is that your friend's professor was responding to a problem common in his recent experience and his caveat was either overstated or taken as gospel when it wasn't meant as such.
Does that help?
I should add:
"Though" and "although" are subordinating conjunctions. They subordinate information. That's a good thing, until someone uses them to subordinate info that should be getting top billing in its sentence.