속에

속에

"Inside" or "in."

The phrase '속에' describes the space physically encapsulated, the forest, housing animals.

숲 속에 동물들이 살고 있어요.

Animals are living inside the forest.

In this sentence, '속에' specifies the inner emotional or metaphorical location, within his heart.

그의 마음 속에 행복이 가득해요.

Happiness is full inside his heart.

Here, '속에' is used to indicate that the book is located within an enclosed space, the desk.

책상 속에 책이 있어요.

There is a book inside the desk.