When you have a teammate who is down in kills and CS, and exp

DO NOT take their CS, stay in their lane and soak exp. destroy the enemy tower then pushed their wave to the enemy T2 tower where his opponent can freeze it there.

I'm freaking tired of this, you are turning a 5v5 into a 4v5 by denying your teammate what he needs to get back into the game

things you could do: gank and get your laner the kill. (or even the assist) ZONE the enemy laner away so your laner can get some CS safely get an objective that provides global map gold poke down the enemy tower (but don't kill it, let your laner take the solo gold from it).