New player

[Some guy on the internet]

Hello everyone
I am new here...I just bought SH4 on steam yesterday and I am really enjoying it...
I heard about this community , so maybe some of you guys can help me with some questions regarding the gameplay...
1) When you engage lone merchants , the AI is capable of calling in for help from nearby warships and airplanes?
2) What is the difference between pasive and active methods of detection?
3) What is the thermal layer? When I submerge I often hear a voice saying "Passing thermal layer"
4) Is there any disadvantages or dangers if you run the submarine at very big depth?
5)After I finished my second patrol I tried to upgrade my porpoise class with a surface radar , but there was no option for it..I was only able to get an air radar
6)Is crew managment important?
7) Is possible to get attacked by enemy submarines?
8) Is there any way to stop the time compression reset when you receive radio messages? I can be very annoying sometimes
That would be everything for now...Thank you
And I am sorry for any typos..English is not my native language..I am trying my best