1. difference between 안 and 속 First, let's look at the example sentences. 괄호 안에 답을 쓰시오. 버스 안에 사람이 많다. 자동차가 터널 안으로 들어갔다. 수박 속이 꽉 찼다. 바다 속에는 물고기가 많다. 보물을 땅 속에 묻었다. What's the difference? '괄호, 버스, 터널' are all have empty spaces. And, '수박, 바다, 땅' are all filled with something. In other words, the first concepts can be described as being empty and full. '안' : inside of something which is empty '속' : inside of something which is full Being full includes not only visible but also abstract, 마음 속에 꿈을 가져야 한다. Like the example above, '마음' is filled with various abstract feelings such as joy, sadness, and hope etc. Therefore, it is natural for '마음' to use '속' instead of '안'. 2. 내 is another form of 안. The usage of 내 is same to 안. 내 cannot be used alone. 내 is usually used after a noun. 안 can be used alone. e.g 안으로 들어오세요. (O) 내로 들어오세요.(X) 내 is more commonly used in formal and written language. 안 is more commonly used in informal and colloquial language.