It is often taught that “between” is used for 2 items and “among” for 3 or more – but this is not completely accurate. The more accurate difference is this:
This example will help illustrate the difference:
Here’s another example:
These sentences are the same – but in the first sentence, we specifically name each of the three options (the green shirt, the blue shirt, and the black shirt) – so we use the word between. In the second sentence, we treat the items as a group (“these three shirts”) so we use the word among.
You can find more information and examples here: Grammar Girl: “Between” Versus “Among”
Which is correct: “Between you and I” or “Between you and me”?
The correct phrase is “between you and me” – never “between you and I” – this is something that even native speakers confuse!
This expression is used when you want the other person to keep some information a secret, for example:
This means you don’t want the other person to tell anyone else your opinion about John’s lack of intelligence/ability.