These two words have a similar meaning. Which one to pick will often depend either on word choice or how it is most commonly used. Lots of exposure to both words in context will help you get used to choosing the most natural choice. ex) Your bank account has a maximum daily transfer limit of $5000. (here, we only use the word 'limit.' ex) This point marks the boundary between these two neighboring cities. (here, we only use the word 'boundary.' ex) There is a limit to my patience. (here, we only use the word 'limit.') ex) Touching a stranger often infringes on her personal boundaries. (here, we only use the word 'boundary.' Hope that helps :)