The relaxed vowel in full lives in a surprisingly small set of words: full, book, good, put, could, would, look, foot, push, cook. Learn that list and you can predict which vowel is coming before you hear it.
Learn the List, Then You Can Predict It
That is a genuinely different strategy from the one ship and sheep asks for, where you have to catch the difference in the moment. Here the spelling carries the answer for you. When you do need to judge purely by ear, the reliable cue is lip shape rather than length: fool pushes the lips forward into a small tight circle, and full barely rounds them at all.
The Pairs Worth Recognising
Because the relaxed vowel lives in such a small group of words, there are only a handful of true pairs — which makes them worth knowing individually:
full / fool
"A full day" and "a fool day": only one of these is a sentence.
pull / pool
Signs on doors, and swimming. Both extremely common.
look / Luke
A name against an everyday verb, so context rarely helps.
soot / suit
Work clothing against fireplace residue, and occasionally a real mix-up.
Notice how short this list is compared with ship and sheep. That is the useful part: the rare vowel doesn't have many places to hide.
Why the Rare Vowel Loses
Korean, Japanese and Spanish do not separate these two vowels, so both arrive as one sound. English then stacks the odds by making one of them common and the other scarce. What makes this pair unusual is that the smaller category is also the more predictable one. Most sound contrasts ask you to train your ear over months. This one mostly asks you to learn ten words.
Nearby Vowels Worth Testing
- ship / sheep
The same tight-versus-relaxed split, and the pair this one is usually taught alongside.
- cat / cut
Another vowel with no clean equivalent in Korean or Japanese, folding toward its neighbour.