Peeking- Pressing left and right on the D-pad (by default) causes you to lean one way or another... allowing you to pick off shots from behind cover.
I Got Shot Through the Door!- Except for 9mm and .4 calibur-type bullets, shots can go through doors. The higher the power of the gun, the more damage it does if it goes through and hits someone. Various steel doors may not be pierced by some weapons, so test the different doors out in the level.
Night or Thermal Vision?- Night Vision works better for close ranges and non-bright environments, Thermal Vision works better for long ranges and through smoke. Unfortunately, both of these can severely impair your view of the environment, so to remain aware, switch them often.
Someone Is On the Other Side of the Door!- You have various options. One is to set a breaching charge on the door. When it explodes, the person on the other side will be messed up. Another thing is to crack it open using up or down on the D-pad (by default), and tossing in a grenade. A third way is to get a high powered gun, and if you have a good idea of where they are, mow them down through the door.
I Jumped to My Death (Ladders)- Press A (by default) when looking at a ladder. If there isn't a "climb ladder" icon on it, then move the targetting cursor around until you find it. Going off of a place that has a ladder usually kills you, whether or not it was a long fall.
I'm Seeing Blurry Stuff in Black and White and There is a Ringing Sound!- You just got exposed to a Flashbang Grenade. If a Flashbang goes off while you are close or looking at it, whether or not you threw it, it's going to mess you up. Bad.
All My Clips Are Not Full!- That's right. This is a realistic game, when you Reload you are simply switching magazines. If you constantly reload when you have enough rounds left in the magazine, you will end up with a bunch of magazines that won't have enough ammo.