Why is my bread so dense and moist?

I have been trying to make bread using a bread mix and my mixer. I read that the amount of water you should add can vary on the humidity at the time. The mix instructions say that I should add 300ml of water to the mix but I find that by the time I have added 250ml it is already quite wet, so I never add the full 300ml. There are no instructions on how long to knead the bread if using a mixer but a recipe for bread in my mixer book suggests around 5 minutes. I have done this but I think that maybe it needs longer.

My question is: Could my bread be too dense and moist due to:

Too much water? Not enough kneading? Not leaving it to prove for long enough?

What is a "warm place"? How warm is warm, without killing off the yeast? As you can see, I'm quite a beginner with baking!