Assuming bread was prepared and baked by a Gentile using only kosher ingredients (e.g. kosher flour, water and kosher yeast) and only kosher equipment, what conditions would render such bread fit for consumption by a kosher-observant Jew Lekhatehhilah? Bedi'avad?
There's an entire chapter of Shulchan Aruch (Yore Dea 112) — sixteen subsections plus all the commentaries and later works — devoted to this, so obviously answers here, including this one, can't do the topic justice. But the Star-K and the OU summarize the main points, qq.v., and I'll briefly summarize the summaries: