Now we have a first one here: For the formation of the present participle there is only one rule and there is no exception to this rule.

To the root of the word -end is appended   Infinitive root present participle (root + end)     essen ess- essend   sagen sag- sagend   trinken trink- trinkend   schreiben schreib- schreibend   wollen woll- wollend