The pronunciation of some vowel pairs is fairly easy to predict, while others are less clear, but the trickiest one of all is the pair [ou] – this vowel pair uses many different pronunciations.
Here are the different ways that [ou] can be pronounced.
Long-O: dough, your, court, though, shoulder, source, four, thorough
Short-o: thought, bought, cough, fought, ought, brought
Long-U: through, routine, group, youth, detour, you
Short-u: cousin, enough, young, couple, rough, country, tough, trouble, touch, southern
Short-oo: could, couldn’t, would, wouldn’t, should, shouldn’t
Vowel /aw/: about, cloud, count, doubt, hour, house, mouth, noun, our, out, round, shout, south, thousand
Schwa for the suffix [-ous]: enormous, nervous, famous, cautious, obvious, various, serious, tedious
R-vowel: courage, journal
And on top of that, one word has two pronunciations: “route” with Long-U and “route” with Vowel /au/.
So, the best advice for [ou] is, first, to learn the correct pronunciation for all of the words here, so that you are confident with these ones. Then, for any new words with [ou] that you find in the future, be ready to listen carefully to the vowel sound.