Correct, @AnthonyCusack - they are called stative verbs, because they are states, not actions; and in Welsh stative verbs in the past have to be in the imperfect rather than the preterite (= simple past). Unlike English, which is what causes the difficulty.
Verbs like perthyn belong and cynnwys contain are similar examples.
In English it’s the PRESENT tense that makes a distinction between stative and action - stative verbs generally don’t use the present continuous. We don’t say: This box is containing three items, any more than we say I am knowing Alfred.
So (generally!) you can tell if a Welsh verb CAN’T be used in the preterite by seeing if the English equivalent CAN’T (generally!) be used in the present continuous! 
Good trick - breaks the ice at parties!! 