I bake my bread for about half an hour. At the 15 minute mark, the bottom is already slightly burnt, but the bread is clearly not done. My oven doesn't have a thermometer, so I don't know exactly what temperature it's at. I preheat it for 15 minutes at the highest setting, and then I drop the temperature when the bread goes in. I also put a container with water at the bottom to create vapor. How do I stop the bread from burning?
it's possible that the heat only comes from the burner/element at the bottom. Especially if you have a gas oven.
Try bufferring the bottom of the bread with a pizza stone. A terracotta clay pot saucer, or practically any unglazed natural stone tile can substitute. Marble or granite are not ok.
It is highly likely that the temperature of your oven is too high. While I would recommend simply buying an oven thermometer, which are not very expensive, if you are in a part of the world where that is not feasible, according to this post at Tip King:
Hopefully, this method will let you estimate the temperature of your oven accurately enough to adjust it within the range that your bread is successful.
Note that I have not verified this method personally.
If this is not the cause of your problems, you will need to describe how you are baking your bread (the recipe, whether you are using a stone, a pan, how you are shaping the loaves, what their size is, and so forth) in much more detail to get a better answer.
Try to put your bread a space more up and in the center of oven. And check if (maybe) the pan is too large for the oven. In this case it wouldn't leave space for hot air to circulate.
This is a matter of examining because the heating acts more on one side than the other. Preheating is irrelevant, because the heat is now distributed evenly throughout the oven.
The problem arises when you put the pan in the oven. The same shape of the pan deflects the heat to the sides of the oven, to raise it again and then gather in the high part.
Usually when the bread, cakes or any preparation made baking are overcooked on the one side, it is because the container is not well positioned in the oven, and is closer to that side. Changing the position of the pot can also compensate imbalances heat inside the oven. But this result can be achieved only through experiencing, every oven is different from any other.