I was always under the impression that the allies had effectively done a hard reset of the German legal and political system and recreated Germany effectively from the ground up constitutionally. But recently in the Böhmermann affair Erdogan made a lawsuit against a German poet using some ancient law that predates ww2. So did some laws survive the occupation. To what extent did the allies govern Germany in accordance with its pre-war legal and constitutional systems. Was there an attempt to do things legally by German standards or did they simply restart the whole government.
pretext: Not a historian, but a german legal professional. I can provide links and sources, if requested. The links provided so far are all in German.
You have to look at it depending on the area of law. There are two major ones in german law: Public Law (Öffentliches Recht) and Private Law (Zivilrecht). Public Law consists of our constitution (Grundgesetz - in short GG), Administrative Law and Criminal Law (nowadays also European Law and International Law).
That said we can look at a historic context: There where two major periodes in german history before WW2 - the Kaiserreich - most influential person here was, at least for this question, not the Kaiser but Otto von Bismarck. And after WW1 the Weimar Republic.
I won't comment on the different developments of the two parts of Germany between 1945 and 1990.
The Kaiserreich era gave us, from a legal point of view, the Bürgerliches Gesetzbuch (BGB) in 1900, which is the main legal text for Private Law and is still in use today - it has been changed due to historic developements, but the basic ideas are still the same. The Nazi era hasn't changed much here.
Even older is the Handelsgesetzbuch (HGB) from 1871, which codified the law for merchants and buisness people, and also still in use.
Public Law might be the area that is more interesting to look at, as it is law wherein the state dictates certain rules to its citizens.
First to mention are laws on social security. Germany und Bismarck were the first country to introduce a wide range of social security laws in 1883 - mostly mendatory health ensurance. The thought was to bind the german workforce to the state, as it was needed for industrialisation, also Bismarck wanted to weaken the SPD (Social Democrats), which is the oldest party in Germany and represented the working class and unions. Those laws are in effect until today, tho there were some changes to adept them to modern times.
Lookin at Criminal Law the code is the Strafgesetzbuch (StGB) which originally was introduced in 1872. They tried reforming it in the Weimar Republic but failed. It is however used until today, tho it has been changed a lot to represent changes in society and worldview. Most prominent example would be views on homosexuallity, which was punishable in the Kaiserreich and Weimar Republic- In the Third Reich homosexuals were put into concentration camps and had their own special sign - similar to the yellow star for Jews, only that it was a pink pyramid on its tip (the so called "Rosa Winkel"). The Article 175 StGB was removed from german criminal law in 1969 - so homosexuality wasn't punishable since.
Very controversial is Article 211 StGB - Murder (Mord). It is an article which was introduced by the Nazis and is still in our criminal code with the same wording. There are pushes to change it, mostly because it was introduced by the Nazis, but so far no one has come up with a new way to change it, that everyone could agree on. Basically the article says that murder is killing a person under special circumstances (Mordmerkmale) and one would not get the idea that it has the special history it does.
Looking at Public Law the most important part - the constitution - is based on the Constitution of the Weimar Republic. Basically the Kaiserreich and the Weimar Republic were polar opposites. The Kaiserreich was very conservative and the Weimar Republic was rather libral and social. The Weimarer Constitution is actually still used in certain areas, mostly on questions of religion (compare Article 140 GG).
Quite interesting is that Germany wasn't really part of international law - such as war on the conduct of war or international criminal law - before 1945. Which is very interesting as the first two major events of international criminal law took place in Germany. After WW1 the victorious parties wanted trials as well - but they left the prosecution to Germany, which did do the job rather half heartedly in the so called "Leipziger Prozesses". More know are obviously the "Nürnberger Prozesse" after WW2, where the Allies took over the prosectuion, mostly based on the experiance of the Leipziger Prozesse.
So yeah, that is the gist of it. One has to point out that central ideas have changed. Germany was rather reluctant to think too much about the Nazi past, after WW2. Mostly people were concerned with surviving and rebuilding. So we employed many of the judges, prosecuters and administrators who had been judges, prosecutors and administrators before 1945. So many of the views were transported to jurisdiction.
There were obviously rules on german military and similar topics. However after Germany was divided the prevailing intent was to rebuild West-Germany and make it strong enough to withstand possible Soviet/Russian agression.
The Allied and occupying forces dictated parts of the Grundgesetz and West-Germany became more a part of the western / european sphere of ideas. Ideas of Humanism, Liberalism and Socialism were the prevailing ideology. But keep in mind that the party in power for most of the time after 1945 was the CDU (Christ-democrates, who are conservative, compareable to the Democrates in the US), so that had a major influence as well.
Germany eventually came around to review its past - mostly after Eichmann was captured by Israel. There were laws to pardon some crimes comitted during the Third Reich - most noticably the so called "Verjährungsskandal"(Scandal of Limitations) of 1968, that basically made accomplices of murder retroactively not prosecutable after 1960. They changed the statue of limitation from 20 years to 15 years.
A postive note might be the "Radbruchsche Formel" that engaged the question how one has to handle law, if it is unbearably unjust or if a law is in violation of the equality of every human being, which is fundamentely part of the idea of law. Radbruch stated that in those cases a judge had to go against the codified law and for justice instead.
There are noticable marks in our legal system, that tell the story of the growth - besides the aforementioned Article 211 StGB or Article 140 GG, it is a clause in our voting laws, that a party must reach at least 5% of the votes, to get into parliament (which is due to experiances in the Weimar Republic) and also the codification of so called "Grundrechte" (basic rights) and a clause barring changes to Article 1 of the Grundgesetz, which states that every human being has a certain inherent dignity, that cannot be touched.
I hope my comment answers your question and that it doesn't hold too many spelling mistakes - it is 5 a.m. over here.