Is there a rule, or rules, for how to pronounce ough?
trough (short o, gh pronounced f), thought (short o, silent gh), though (long o, silent gh), through (oo, silent gh), tough (short u, gh pronounced f).
There are at least five ways to pronounce ough:
As for the rules, the only one I can think of is that the pronunciation found in thought only happens with the ough is followed by a "t". Other than that, there's really no way to tell by looking at the word.
I remember the first time I met someone with the last name of Gough. I didn't know if it was pronounced as "go", "goff", "guff", or "gow" (that last one rhyming with "now," not "know").
English has many letter combinations like this; consider:
to name but a few. Some words (like bow and wind) have vowels that can be pronounced two different ways, depending on the meaning of the word, leading to some ambiguous sentences, like this one:
He took a bow as he gave her a bow.
I think it is just one of those things in English that doesn't necessary have a consistent set of rules. You just have to learn them individually. This my perspective as a native English speaker. Someone approaching English as a foreign language may have developed a system of remembering these things, but this is outside my experience.
There isn't a rule, really. English is known for it's special spelling at times. There's a dummy word that perfectly describes this:
Ghoti, a respelling of the word fish and pronounced the same way (/ˈfɪʃ/). It uses the sounds gh from enough, o from women and ti from intention (not necessarily those words, but those sounds).
It just goes to show that the English pronunciation can be quite unpredictable at times if your point when you're basing it on spelling. So speaking the language correctly is just a matter of studying the pronunciation of tthe words or relying on your language instinct.