Also, the irregular endings follow the same pattern with some minor alterations. This part is pure memorization. Here is where the exceptions start. Here are those lovely stem vowel changes. Remember, these are the exceptions to the rule, and do not occur very often.
Memorization is mandatory.
These verbs also have stem changes in the second and third person singular. Luckily, this change is an umlaut over the a. And here is the wonderful list of these very special verbs. And here are the -n verbs. These verbs are ones that you absolutely must memorize.