German verbs : stems and conjugations
1. Verbs are really the key words in the grammar of any language, since any proper stnetnce must contain a verb, and it is the verb which makes clear the relationship between the noun phrases in he sentence.
2. The base form of any verb, the form under which it appears in any dictionary, is called the infinitive, and in German it nearly always ends in " -en", eg, füllen (to fill), kaufen ( to buy).
3. Notice that the English equivalent begins with " to", which is the hall-mark of the infinite from in English.
4. In German, the " -en" may be called the infinitive ending, and what comes BEFORE it is the " verb stem", so " füll-" , " kauf-", are stems on to which OTHER endings will be ADDED to the various parts of the verb.
5. The rule that German infinitive end in " -en" has as exceptions ONLY verbs whose stem ends in " -el" or " -er", such as lächeln ( to smile), flüstern ( to whisper) and a tiny number of odd verbs such as sein ( to be ) and tun ( to do) -- in any case, ALL German infinitives end in " -n".
6. The list of all the possible endings which may be substituted 代替 for the infinitive ending " -en" constitute the conjugation of the verb, corresponding to the declension of a noun.
excerpted from German Grammar by Norman Paxton, ch 11, page 57.