The participle: ge- -t, ge- -en, and the ones with no ge-

The participle is built from three pieces: ge- in front, the stem in the middle and -t or -en behind. Regular verbs give ge- + stem + -t: gespielt, gemacht, gekauft. Strong verbs give ge- + stem + -en and nearly always change their vowel: sprechen gives gesprochen, trinken gives getrunken. And two families take NO ge-: those ending in -ieren and those starting with be-, ver-, er-, ent-, ge- or zer-.

The usual slip. Putting ge- on everything. gestudiert is what comes out by itself, and the right form is studiert. The reason is not grammatical but phonetic: ge- only goes on when the stress falls on the FIRST syllable. In studieren it falls at the end and in verkaufen on the stem, so in both cases the ge- has nowhere to sit.

Why it catches you out. English has exactly the same split —walked against spoken, written, taken— and the -en ending of the strong ones is the same -en as German's. So half the work is done. What English has no trace of is the ge-: it lost it, though it is still hiding inside the word «enough», which used to be «genoh».