Some verbs don’t add ge- to the beginning to form a past participle.
The reasons for this could be one of the following:
they end in –ieren, eg studieren > studiert (however, apart from not starting with ge-, these verbs are regular)
they start with an inseparable prefix such as be-, ent-, er- or ver-, eg verstehen > verstanden
the main verb already starts with ge-, eg gewinnen (to win) > gewonnen (won)
Examples in use
Sie hat 100.000 € gewonnen. - She won €100,000.
Wir haben nichts verstanden.- We understood nothing.
Mein Bruder hat in Passau studiert. - My brother studied in Passau.
Separable verbs
Separable verbs consist of two parts - a main verb and a preposition, eg:
auf + räumen > aufräumen (to tidy up)
an + ziehen > anziehen (to put on)
auf + stehen > aufstehen (to get up)
In English, we leave both parts separate in the past tense:
he tidied his room up
she put her coat on
In German, both parts have to be put back together to make the past participle, with ge- sandwiched in between, rather than added to the beginning as with regular and irregular past participles, eg: