To form the past tense in German, you need the conjugated auxiliary verb (haben or sein) and the past participle of the verb, which comes at the end of the sentence. For example, if you want to say "I have gone", it would be:Ich bin (auxiliary verb) gegangen (past participle).In this case, because the action involves movement, you would use 'sein' over 'haben'. The past participle is formed by taking the infinitive of a verb, replacing the '-en' with '-t' and adding 'ge' at the start. As with most grammar, there are exceptions to this rule; certain verbs instead retain the '-en', and there are some that do not add a 'ge'. These rules are unfortunately just something you learn with time!