Can someone point to some strict rules or rules of thumb for using être/avoir for passé composé for irregular verbs? Does it follow from change of radical for different tense?
In German, a change of the radical vowel means, you have to use avoir. But there are also some verbs, for which one can use either avoir or être to build Perfect, or at least one usage is more common in the south and the other in the north region of Germany. A rough rule in German and in general Germanic languages is that "weak verbs" and intransitive verbs mostly need être to build the perfect as far as I know. But French has 3 times as many irregular verbs as German, so maybe there are very different rules. Are my hints and assumptions above a good start?
PS: This site lists the rules for German verbs very accurately (in German).