it is good to put your sentry in places your enemies least expect you to like in the water and in corners
spies are your worst enemy and pyros are your best friend
the black box is a weapon for soldier that will give you 1 hp back for every 4.5 damage you do to an enemy with it
pressing U is how you send a massage to only your team, it is a good to make plans with your team with the enemies knowing (i have used it before to tell my team to set up camp in the enemy sewers and to bulid a sentry near our intell)
heavy is the tankiest class out of all the classes with 300 health
sub-classes are ways to play classes in pretty different and cool ways (exampe: demokights use a sword and shelid instead of their normal weapons. the shelid replaces the sticky bomber which now allows you to charge at your enemies with your sword. there are 3 different shelids to choose from)) there are many videos on all the different sub-classes you can play with
also add me on steam (if you haven't already) my user is necomancer