they are similar. decidedly just means resoluteness. decisive means means it meant the decision between success or failure, "the decisive moment" = the moment which decides the outcome, a "decisive victory" = a deciding victory (i.e. it decides the final outcome )， we usually only use 'decisive' to describe the extent of a major or very successful victory (usualy in war, or sports). 'decidedly' is not very common, it would just mean someone was extremely confident, or very resolved. decidedly = confidently, resolutely. (決意地) decisively = usually used to mean: with complete success, thoroughly successful without question or difficulty. (often used with 'victory': the enemy won the battle. it was a decisive victory that changed the outcome of the war - nobody could argue otherwise.) （決然地？） In spoken language, we don't use either of these words very often, you will likely only see them in written english or television broadcasts.