The גזירה lest one come to eat it with meat (שמא יבוא לאכלה עם בשר), the exceptions of a small quantity (כדי אכילה בבת אחת) and a distinct shape (שינה צורת הפת), the oven smeared with fat (אליה), and baking bread, roast and fish together with meat (ריחא) (Shulchan Aruch, Yoreh De'ah 97 — 3 se'ifim)
אֵין לָשִׁין עִיסָּה בְּחָלָב, שֶׁמָּא יָבוֹא לְאָכְלָהּ עִם בָּשָׂר; וְאִם לָשׁ, כָּל הַפַּת אֲסוּרָה אֲפִלּוּ לְאָכְלָהּ לְבַדָּהּ. וְאִם הָיָה דָּבָר מוּעָט כְּדֵי אֲכִילָה בְּבַת אַחַת, אוֹ שֶׁשִּׁנָּה צוּרַת הַפַּת שֶׁתְּהֵא נִכֶּרֶת — מֻתָּר.
One does not knead dough with milk, lest one come to eat it with meat; and if one kneaded it, the entire bread is forbidden, even to eat it alone. But if it was a small quantity, enough to eat at once, or one changed the shape of the bread so that it be recognizable — it is permitted.
Shulchan Aruch, Yoreh De'ah 97:1LEVEL 01
Hebrew text of the 3 se'ifim with a fluent English translation. The גזירה lest one come to eat it with meat, the two הכרים (small quantity, distinct shape), the oven smeared with fat (אליה) and joint baking (ריחא) explained with practical cases.
📖 StudyLEVEL 02
In-depth pilpul: the nature of the גזירה (a prohibition by decree, not by בליעה), the limits of שינוי צורה per the Mahari"t (at home, and only at baking time), אליה / שמנונית בעין and the need for ליבון (Taz), the bread's נ"ט בר נ"ט (Pitchei Teshuva sk4), חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (dough with milk forbidden / permitted, small quantity or distinct shape, smeared oven before / after ליבון, small oven / large open oven), golden rules, classic pitfalls (גזירה, אליה בעין, ריחא, נ"ט בר נ"ט) and memorization of the 3 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".
📖 StudyAccording to the Shulchan Aruch (YD 97:1), one does not knead dough with milk (אין לשין עיסה בחלב), by גזירה, lest one come to eat it with meat (שמא יבוא לאכלה עם בשר). If one did knead it, the entire bread is forbidden, even to eat it alone. The decree is nonetheless lifted in two cases: a small quantity, enough to eat at once (כדי אכילה בבת אחת), or a distinct shape (שינה צורת הפת) that makes it recognizable one does not eat meat with it. Hence the Rama notes the custom to knead bread with milk on Shavuot. For practical halacha, consult your Rav.
Because this is not a matter of actual absorption (there is no meat in the bread), but a גזירה — a rabbinic fence. The Shach explains: שמא יאכלנה עם בשר — we fear one may eat this dairy bread with meat and transgress basar be-halav. To prevent any confusion, the Sages forbade the bread itself, even eaten alone. The prohibition is thus by decree, and not by an actual mixture. For practical halacha, consult your Rav.
According to the Shulchan Aruch (YD 97:3), bread baked together with a roast, or fish roasted in the same oven with meat, are forbidden to eat with milk — because of the odor (ריחא) that transfers the taste. This applies in a small oven; but in a large oven holding twelve esronim, whose mouth is open, it is permitted (the odor dissipates). Likewise if the roast is covered, or a פשטיד\"א whose opening is covered: permitted even in a narrow oven. See siman 108 for the customary practice. For practical halacha, consult your Rav.