The bones of the prohibition that combine (or not) with the permitted to nullify it, the rule אין מבטלין איסור לכתחילה (one may not nullify a prohibition deliberately; שוגג / מזיד), חוזר וניעור (the prohibition that 'reawakens'), how one estimates (רוטב, קיפה, מין במינו) and the משהו absorbed in a vessel used in abundance (Shulchan Aruch, Yoreh De'ah 99 — 7 se'ifim)
אֵין מְבַטְּלִין אִסּוּר לְכַתְּחִלָּה; וַאֲפִלּוּ נָפַל לְתוֹךְ הֶתֵּר שֶׁאֵין בּוֹ שִׁעוּר לְבַטְּלוֹ, אֵין מוֹסִיפִין עָלָיו הֶתֵּר כְּדֵי לְבַטְּלוֹ. עָבַר וּבִטְּלוֹ אוֹ שֶׁרִבָּה עָלָיו — אִם בְּשׁוֹגֵג, מֻתָּר; וְאִם בְּמֵזִיד, אָסוּר.
One may not nullify a prohibition deliberately; even if it fell into permitted food that does not have the measure to nullify it, one does not add permitted food in order to nullify it. If one transgressed and nullified it, or added: by error (שוגג), it is permitted; deliberately (מזיד), it is forbidden.
Shulchan Aruch, Yoreh De'ah 99:5LEVEL 01
Hebrew text of the 7 se'ifim with a fluent English translation. The bones that combine (or not) with the permitted, raw vs cooked, the rule אין מבטלין איסור לכתחילה, שוגג / מזיד, חוזר וניעור and the משהו absorbed in a vessel explained with practical cases.
📖 StudyLEVEL 02
In-depth pilpul: the sugya of Chullin, the מחלוקת Shach / Ran on soft vs dry bones, גוף הקדירה that does not combine, "כמו שבא לפנינו" and estimation in מין במינו, the yesod of אין מבטלין (דרבנן according to the Shach), שוגג / מזיד, חוזר וניעור, חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (bones to the prohibition / to the permitted, raw / cooked, שוגג / מזיד, חוזר וניעור or not, כלי בשפע / כקערה), golden rules, classic pitfalls (אין מבטלין, מין במינו, כלה מחמת האור) and memorization of the 7 se'ifim.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Megadim and Pitchei Teshuva, then the Sephardic poskim (Yabia Omer, Yalkut Yosef, Or LeTzion) and Ashkenazi poskim. Note: the Shulchan Aruch HaRav does not deal with this siman — this is a level of psak, not "Daat HaRav".
📖 StudyNo. The Shulchan Aruch (YD 99:5) rules that one may not nullify a prohibition deliberately (אין מבטלין איסור לכתחילה): even if the prohibition fell into permitted food that does not have the measure (60) to nullify it, one does not add permitted food in order to nullify it. If one transgressed and nullified, or added: בשוגג (by error) → permitted; במזיד (deliberately) → forbidden for the one who nullified and for the one for whom it was nullified (and it is forbidden to sell it to another Jew), but permitted for any other person. For practical halacha, consult your Rav.
This is the principle of se'if 6 (according to the Rama): a prohibition already nullified (there were 60 against it) to which one then adds more of the original prohibition (מן האיסור הראשון) 'reawakens' (חוזר וניעור) and becomes forbidden again — whether מין במינו or מין בשאינו מינו, dry or liquid, whether one knew of it in the meantime or not. By contrast, a kazayit of milk that fell into water and was nullified by 60, then fell from the water into a meat pot, remains permitted even though there are not 60 against it in the meat, since it was already nullified in the water. For practical halacha, consult your Rav.
According to se'if 1, a piece of נבילה containing meat and bones that fell into a pot of permitted food: the bones of the prohibition combine with the permitted to nullify the prohibition (being inedible, they are not 'of the prohibition'), but the marrow (מוח) in the bones combines with the prohibition, and the body of the pot combines neither with the one nor the other. The Rama brings stringent opinions (in case of loss one relies on the lenient), and the Shach distinguishes soft bones (with marrow/moisture → with the prohibition) from dry bones (→ with the permitted). For practical halacha, consult your Rav.