Let's start with let's. Except for some set phrases (example: "Let us pray") let us is almost always expressed as a contraction: let's.
Example:
- Let's go!
It is almost always used to express an imperative, thus:
- Let's do it!
However, sometimes it is used to express something like an admonition, thus:
- Let's do what we ought to do.
Sometimes it is used to make suggestion, thus:
- Let's keep this just between us.
