A regular or normal verb is not identified by how it looks, but how it behaves. So, how does a normal verb behave?
1. It does not conjugate. The verb stays the same in all the tenses.
2. It is not separable. It stays the same in all the tenses.
3. In the past tense, the GE- prefixed is added to the verb if it does not already have a prefix. That's it.
Remember the prefixes? [ge-, be-, ver-. her-. and ont-]. So. the verb hou will become gehou in the past, but the verb onthou will stay onthou in the past (no geonthou please).
Now that you theoretically get it, let me explain .. Regular verbs consist of root verbs on the one side and prefixed verbs (root verbs with a prefixed on), and inseparable compound verbs on the other side.
Root verbs get the GE- prefix in the past tense, and both prefixed verbs and inseparable compounds do not get the GE- prefix in the past tense. That is it.
Remember, practice the present -- past -- future till you can do it in your sleep.