The "oo" letter combination has three common sounds and a couple other less common sounds. The rules for the double-o will not help you learn the sound, there are too many exceptions to the rules.
Here are some short "oo" words:
- look, book, took, foot
- wood, good, understood
Here are some long "oo" words:
- too, zoom, tool
- shampoo, moon, goose
There are a few exceptions: R-controlled oo words: door, floor
Here are some other "oo" words:
- flood, blood
- coordinate, cooperate
- zoology, microorganism