EXCEPTIONS

in the formation of Partizip II

Verbs whose stem ends with -t

For verbs whose Stamm ends with a t, an e is inserted to separate the two t's.

Examples
warten --> gewartet
antworten --> geantwortet
heiraten --> geheiratet

Foreign verbs ending with -ieren

For foreign verbs in the German language ending with -ieren, the Partizip II does not take the ge-. The ending is still t as for other regular verbs 

Examples
investieren --> investiert
studieren --> studiert
akzeptieren --> akzeptiert

Unusual irregular verbs

The Partizip II of irregular verbs usually have -en as ending. However some irregular verbs will have a change in the stem while retaining the -t ending of the regular verbs. For this reason, they are sometimes referred to as 'mixed' verbs. It is a probably easiest to remember them as irregular verbs and commit them to memory. 

Examples
wissen --> gewusst
denken --> gedacht
bringen --> gebracht