Here's some difference I found between the Little Mermaid book, and movie, here they are.
1. In the movie, The Little Mermaid has a name (Ariel, but in the book, she's just called The Little Mermaid.
2. A girl finds the prince on the shore after the Little Mermaid saves him, but, in the movie, Max and Grimsby finds him.
3.In the book, The Little Mermaid has 5 sisters, and lives with her father, and her grandmother. In the movie, she has 6 sisters, and only lives with her father.
4. The sea witch cuts the little mermaid's tongue off, instead of, like in the movie, putting it in a necklace.
5. When the little mermaid will get legs, it will feel like a sword has gone through her, and every time she takes a step it will feel like she is stepping on swords as hard enough to make her bleed. In the movie, this is never mentioned.
6.The prince ends up marrying a different princess and marries the princess, and not the little mermaid. When in the movie, the prince marries Ariel.
7.If she doesn't get the love of the prince, she will turn into sea foam, considering that merfolk, do not have souls. In the movie, if she doesn't get the love of the prince, she will belong to the sea witch.
8. The sea witch doesn't try to interfere with the little mermaid and prince's relationship, unlike in the movie, she is.
9.The Little Mermaid turns into a daughter of the air instead of sea foam because she strove with all her heart to gain a soul. With many good deeds by helping children, she will finally gain a soul. In the movie, the Little Mermaid, never has a chance of turning into sea foam.