If you're trying to be really quiet, moving at 2kts gets you detected a lot less than 3kts (the default for 'ahead slow').
Don't play games with the planes, they miss badly often, but you stop laughing very quickly when they do hit.
Don't shoot the planes down when they are coming directly at you
Don't waste your torps on the convoy escorts.
Know the water depth. If you ask water depth while enemy warships are close by they will hear the ping and search for you. (the water colour on the nav map is a fairly good indicator)
Batteries refill slower than your oxygen supply. I generaly move at 2kts submerged, you can go for at least 24hrs before your air supply gets short, and when you surface your batteries and air should be filled up at about the same time.
If you get inside a convoy the merchants noise will cover yours, allowing you to move at the same speed as them.
Know how deep your boat can go, and remember that this depth will decrease if you take any serious damage (look at the hull integrity bar if you have it)
If a ship you're following violently explodes you may be in a minefield (a lesson I learnt the hard way
)
Hope that helps, also keep reading these forums and you will learn lots of important bits. Experience is a good teacher too
