Simple Rule To Use 退屈 and つまらない
It is the common sense using 退屈 when we get bored. On the other hand, do you explain what つまらない means? つまらない is also used as the meaning of “bored”. So that’s why some Japanese learners have been confused. However, Natives use both 退屈 and つまらない depending on how situations are. In this time, let me introduce what the difference between both 退屈 and つまらない are.
I’m bored because I have nothing to do. / Leisure
退屈 means “There is nothing to do” and “just waste of time”, even if someone wants to do something or is interested in it such as a lesson and a job because he/she is done or knows most of them, so he/she has to spend remaining time of the lesson or the job or etc. For example at the hospital where someone sometimes has to wait quite a long time before seeing a doctor and this is what one of the wastes of time is. That means someone has to spend a lot of time before doing something. So that’s why he/she got 退屈(bored).
I’m bored because it is uninteresting. / Snooze
Unlike 退屈, つまらない is used as the meaning of that someone doesn’t want to do something anymore because he/she is not interested in it. For example, as parents push something to you, someone pushes you to listen to the music which is not your type and so on. So that’s why he/she feels つまらない(bored).