Remember the Modalverben? You learned how to form the Präteritum for these verbs in the last semester. Try to recall the Präteritum of the Modalverben and then click on the following table to see if you are right.

As you can see, for some of the Modalverben, the stem actually changes though the Präteritum has the -te ending like the regular verbs. There are some other verbs that form the Präteritum like this as well. Below are some examples:

The change in stem is a feature of irregular verbs, yet the -te in the Präteritum is typical of regular verbs. That's why they are called mixed verbs in some grammar books. In actual fact, it is easiest to remember them as irregular verbs. And you will find them in the list of irregular verbs.

Other verbs in this category are e.g.: