Courage has been seen doing things that are humorously improbable, such as screaming so loud that it can break the entire basement floor and form a deep chasm as seen in Ball of Revenge, shatter the sun as he did in The Magic Tree of Nowhere, or use various visual transformations in order to communicate, such as turning into a snake, a club and a dragon, as well as many other creatures and objects pertaining to the thing he tries to explain. However, this is only occasional and is assumed to be used for comedic effect.
Although he lives the typical life of an average dog (helping around the house farm and being his average self) he's shown he's pretty quick on his feet when trouble strikes and he's forced to solve the problem in some way.
Courage has shown he can speak English. He demonstrates this when he does things such as yelling Muriel's name in distress or saying his catchphrase "The things I do for love." Unfortunately, this skill falls apart when he's distressed and in turn when it is needed most as he speaks in his signature gibberish when he's trying to explain danger to his owners.