 These have long vowel stems in the present/future tense, which already ends with "-a" or "-ä".These verbs drop the "a" which is present in the present tense stem and replace it with "-t" in the first infinitive stem followed by the standard "-a" or "-ä" first infinitive marker.The "a" dropping to "t" weakens a preceding "k", "p" or "t" so that a weak grade is seen in the first infinitive form.