Thai builds abstract nouns with two prefixes, and the split between them is clean enough to be a rule rather than a list to memorise.
การ goes on a verb of action and gives you the activity. ความ goes on a stative verb, which is to say on what you used to call an adjective, and gives you the state or the quality.
- การเดินทางkaːn dɤːn.tʰaːŋ
travelling; travel
- การทำงานkaːn tʰam.ŋaːn
working; the work of doing something
- ความสุขkʰwaːm sùk
happiness
- ความรักkʰwaːm rák
love
It is the week 10 test, run backwards
If you cannot decide whether a word is an action or a quality, the prefix a native speaker puts on it will tell you, and they will not hesitate about it.
More usefully, it runs the other way: from a verb you already have, you can manufacture the noun and be right most of the time. That is vocabulary you were never taught and did not have to store.
The honest part
There are frozen cases where the pairing does not follow the rule, and a handful of words that take both prefixes with different meanings.
The rule is a productive default, not a law. When a particular word resists it, stop deriving and learn it whole. That is true of every productive pattern in this course and it is not a failure of the pattern.