Ever since Lin Yuelan became Doctor Zhang's grand disciple, she began to 'learn' how to read and write while reading those thick medical books.
Some of the ancient characters were not that different from modern characters. However, there were some that Lin Yuelan needed help with. Even if she was gifted, she was still a human. She wasn't born with the world of knowledge.
Ancient Chinese Characters had a lot of shapes, and they were not spelled out like pinyin. There were two ways one would teach language. The first was through verbal teaching, and the second was with a method called Zhi Yin. The second method allowed people to pick up the language without the help of others, but the premise was the student had to be able to recognize the character's Zhi Yin.
Lin Yuelan was from the modern world, so naturally, she wouldn't understand Zhi Yin. Therefore, she had to ask Jiang Zhennan to teach her verbally.