Just a quick vocabulary lesson today!
There are many ways to classify mitzvot, and one of the major ways is by source: from the Torah or from the rabbis.
Mitzvot d’oraita (mitzvos d’oraisa) are mitzvot directly from the Torah, both the Written Law and the Oral Law. The theoretical punishment (when there was/will be a Sanhedrin) for breaking these mitzvot is stricter than those d’rabbanan. Generally, there are less leniencies allowed, so it’s sometimes important to know whether a mitzvah is d’oraisa or d’rabbanan.
As you can probably guess, mitzvot d’rabbanan (mitzvos d’rabbanan) are derived from rabbinic sources. There can be a much greater variation of observance with mitzvot d’rabbanan than mitzvot d’oraisa. This is where you find the rabbinic “fences” that are intended to prevent people from accidentally violating mitzvot d’oraisa. The rabbis can and do still institute new mitzvot. It’s totally dated by now, but I always think of when electricity and cars were invented.
Related to this discussion are minhagim (plural of minhag, custom). Customs are considered a subset of mitzvot d’rabbanan because they are adopted by the rabbis as mitzvot, but they are less clearly connected to the Torah.
If you want to read slightly more in depth on the subject, check out this good beginner’s reference at Jew FAQ!
This is below your usual high standards. Cooking (via a liqud), eating, and deriving any benefit at all from (kosher domestic) meat with (kosher) milk products from any animal at all is prohibited d'oraita. Lot's of laws that were only written down when the Mishna were codified are nevertheless halachot l'moshe misinai (halacha given to Moses (rhetorically) on Mount Sinai. For example there is no definition of what tefillin are anywhere in the written torah, but the laws of what constitute tefillin are mostly d'oraita.
Furthermore, while the commands to keep and guard Shabbat are d'oraita, Shabbat is one of the best examples where the rabbis (in their own words) built a mountain (of d'rabbanan laws) suspended by a hair (of d'oraita).
I'd suggest you go back and review what constitutes d'oraita, d'rabbanan, halachot l'moshe mi sinai, divre sofrim, etc. Incidentally, I make this recommendation because I am impressed by the depth of your knowledge – this is way beyond the level a pre-convert actually needs to know.
If I understand your last comment correctly, thank you 🙂
But as for the rest, thank you. I'm surprised it took this long for me to make a major flub. This is what happens when you've been left to self-study for 7 years because no one will help you!
Corrections made. Let's hope they're right!
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You might be interested in reading The Oral Law by Rabbi Schimmel for more on this topic.