German/Grammar/Stem-changing verbs

Stem changing verbs
Before moving on to other aspects of grammar, it would be a good idea to expand our repertoire of verbs. In most languages, the best verbs, in other words the ones that are used the most often and are the most important to learn, are usually irregular in some way. So in this section we'll cover a class of slightly irregular verbs, verbs that are irregular but in a regular pattern.

Terminology
In English, we usually say that a verb that does not follow the usual conjugation pattern in irregular and leave it at that. German adds an extra dimension to this by also classifying verbs as weak and strong. This mostly has to do with the way the verb forms its past tenses and the reason lies deep in the history of the German language.

In early German, the past tense was formed using what is called an ablaut, meaning a change of vowel. We can see this pattern in some irregular verbs in English: "swing" ― "swung", "get" ― "got", "bite" ― "bit". At some point the method of forming the past tense changed; verbs that existed in language before then were grandfathered in with the old ablaut method, while new verbs, either borrowed from other languages or converted from other parts of speech, used the new method which changed the ending. The old ablaut verbs are what we now call strong, and the newer ending-change verbs are what we now call weak. English, since it's a Germanic language, follows this same pattern to some degree, as seen in the examples above. But English grammarians usually don't make the strong/weak distinction since the pattern is not as pervasive as it is in German, and since English conjugation is simpler in general than it is in German.

Strong verbs are often classified into types according to the specifics of the ablaut in the Germanic root. But that won't be covered here since it's debatable how useful this information is to people wishing to learn German as it's spoken today rather than how it was spoken in the ancient past.

The stem-changing pattern
We haven't covered the past tense yet, so why talk about strong and weak now? The reason is that there is a subclass of strong verbs which also experience a vowel change in the present tense, and it's these verbs which we'll be covering in this section. An easy example is schlafen ("to sleep"). It's conjugated as follows, with the vowel changes marked in bold.

The general pattern is:

The vowel changes are marked with an asterisk (*) here. Note that it only occurs in the second person familiar singular (the du form) and in the third person singular (the er/sie/es form).

Examples
Here is a list of common stem-changing verbs along with their vowel changes:
 * schlafen ― "to sleep" ― -a- to -ä-
 * Ich schlafe. Du schläfst.
 * sehen ― "to see" ― -e- to -ie-
 * Ich sehe. Du siehst.
 * fallen ― "to fall" ― -a- to -ä-
 * Ich falle. Du fällst.
 * laufen ― "to run" ― -au- to -äu-
 * Ich laufe. Du läufst.
 * sprechen ― "to speak" ― -e- to -i-
 * Ich spreche. Du sprichst.

Dictionary entries
Because vowel changes, if they occur at all in the present tense, occur so consistently in du and er/sie/es forms, the common practice in dictionaries is to list only the third person singular in the present tense. Knowing the common pattern, you can usually deduce all forms of the present tense from the infinitive, which will be the form where the verb is listed, and this one additional form. Even if you you're using a dictionary that lists every possible conjugation, such as the English Wiktionary, it will certainly reduce the load on your memory to learn only one entry in the conjugation table instead of seven.