Leave means to allow to remain or to go away.
- If you leave the book with me, I’ll be sure to read it.
- If you leave quietly, no one in the library will be disturbed.
Let means to allow or to permit.
- Please let me help you with your chemistry homework.
Note: When leave or let is used with the word alone, these words are interchangeable.
- Leave (let) Gerry alone while he is doing his homework.