fix

course8/course_en.md

@@ -122,6 +122,11 @@ struct LinkedList[T] {
 }
 ```
 
+`Node[T]` is a generic struct that represents a node in a linked list. It has two fields: `val` and `next`. The `val` field is used to store the value of the node, and its type is `T`, which can be any valid data type. The `next` field represents the reference to the next node in the linked list. It is an optional field that can either hold a reference to the next node or be empty (`None`), indicating the end of the linked list.
+
+`LinkedList[T]` is a generic struct that represents a linked list. It has two mutable fields: `head` and `tail`. The `head` field represents the reference to the first node (head) of the linked list and is initially set to `None` when the linked list is empty. The `tail` field represents the reference to the last node (tail) of the linked list and is also initially set to `None`. The presence of the `tail` field allows for efficient appending of new nodes to the end of the linked list.
+
+
 ### Adding Elements
 
 ```moonbit