Maybe..these word mean same thing. but I think some different between 심심하다 and 지루하다. When we have nothing to do, we use 심심하다. When we are boring, although we do something(like job, study, washing dish..etc) We use 지루하다. Example sentence in Korean. 할게 없어서 너무 심심해. 공부만 했더니 지루하다. Verb match. 심심하다. bored 지루하다. boring, bored, dull, tedious, tiresome, wearisome

I'm not sure whether this is correct, but i tend to distinguish between the meanings this way: 심심하다 - to lack stimulus. 지루하다 - to be unfulfilled by stimulus. I tend to avoid comparing these Korean verbs with adjectives such as boring or bored. The korean verbs are different notions. Whereas bored and boring are two sides of the same coin. Similar to Korean, English can uses Verb stems to create related adjectives. In this case, the base infinitive 'bore' The -ing participle signals the cause of a state, whereas the -ed participle signals a state as a result of a cause. In other words, cause (ing) and effect (ed).

심심하다 - You are bored and you've nothing to do. 지루하다 - Bored of it.