In German, some verbs have their verb stem preceded by a verbal prefix such as auf-, zu-, mit- etc. They can be called particle verbs.
When the particle is accentuated, it separates from the stem, it is then called a separable verbal prefix.
At the infinitive, the particle is attached to the verb, but as soon as the verb is used in an enunciative or interrogative sentence, the particle is detached from the verb (which remains in the first or second position of the proposition). It is often used to clarify the verb, to qualify the action, to accentuate it, to indicate a movement, or to slightly modify its meaning, for example:
At the imperative, the verb is in the first position and the particle is placed at the end after all possible additions to the verb, for example :
Finally, in Perfekt – or Participle II – the structure is a bit particular. The element -ge- is inserted between the particle and the verbal base, for example :
When a verbal prefix is not accentuated it is never separated from the verb. For example, miß (mißfallen – to dislike) and zer (zerreißen – to tear up) never separate from the verb. Therefore, it is very important to learn this characteristic when you encounter a verb in German for the first time. However, these verbs are in a minority in the German language.
Therefore, in Perfekt – or Participle II – the mark -ge- is not added to the verb.
To go further, do not hesitate to watch this very instructive video.