Sorry! You have to learn this for each word. All of them. It can be an F sound: laugh, enough It can be silent: though, through, eight It can be G: ghost, spaghetti ...and so on, and so forth...

EnouGH rhymes with 'stuFF', bouGH rhymes with 'wOW!, throuGH rhymes with tOO, thouGHt rhymes with sORt, weiGHt is said like 'wAIt, and hiGH is HI ! What a nightmare! Quite ghastly, in fact. GHastly is a hard G .. like in good or gas. Rhymes with 'vastly' I'm sure there are other ways to pronounce GH. It all depends on the word you find the GH in! It's probably easier to learn the words, repeat, and remember, than to learn any rules about pronouncing them. I don't even know if there are rules about it! But I'm sure others here will quote them if there are rules.