If the stamm ends with -t, -d, -m, -n, as in arbeiten (to work), atmen (to breathe), rechnen (to calculate), finden (to find), an euphonic -e is added before the endings of the second singular and plural persons and of the third singular person.
As an example, here is the conjugation of arbeiten:
Ich arbeite I work
Du arbeitest You work
Er / sie / es arbeitet He / she / it works
Wir arbeiten We work
Ihr arbeitet You work
Sie arbeiten They work
But, if stamm’s final -m or -n is preceded by -l or – r respectively, as in learnen (to learn) and qualmen (to smoulder), the previous particularity doesn’t occur and the regular conjugation endings are used.
If the stamm ends with -s, -ß, -x or -z, the ending -s is omitted in the second singular person:
Ex: the second singular person of reisen (to travel) is du reist (you travel) and not du reisst
In regular verbs whose infinitive form doesn’t end with -en but just with -n, as dauern (to last), the ending -e is omitted in the first and third plural persons:
Here is the conjugation of dauern, whose stamm is dauer:
Ich dauere I last
Du dauerest You last
Er / sie / es dauere He / she / it lasts
Wir dauern We last
Ihr dauert You last
Sie dauern They last
If the stamm ends with -l, the first singular person changes, as in sammeln (to collect), whose stamm is sammel.
The first singular person is: ich sammle (I collect)