Present tense

When the stem of a verb ends in  -d, -t, an "e" is added in the 2nd and 3rd person singular and in the 3rd person plural.

The same applies to verbs whose stem ends in a consonant + -m, -n. Exception: When an  -r precedes the -m or -n, like in verbs formen, wärmen, lernen, turnen.