"I" is for Imperative.

In every one of these sentences, I am telling you what "you should" do:

(You should) Look out for the falling rock!
(You should) Use the car key to start the car.
(You should) Head it!
(You should) Dance like no one is watching.
(You should) Mix 3 parts of corn to one part of oil.
(You should not) Don't walk on the grass.
(We should) Let's (let us) have a party.

Each of these has a very different time and place it is used. Sometime, we are happy to be told, say, that a rock is falling on our head. Other times, we are OK with people telling us how to start the car or make a meal. It is because they know more than we do. And sometimes, as with a person watching a ball game, or a friend suggesting a party, we have the right to say "you should" to the other person.

Times NOT to use the Imperative
Sometimes, people who are just learning a language get in trouble with the Imperative. If you are eating and you say "Make me a sandwich," it may seem to that person that you think you have power over him. Most people do not like this feeling. (They might just say, "OK, you're a sandwich."...:)

For that reason, we usually "soften" our talk with "Please make me a sandwich." Or, better, "If you are not busy, can you make me a sandwich?"

The best times TO use the Imperative:

1. When there is danger. We don't have time for anything but the verb, and maybe the reason we are saying it quickly.
2. When the other person wants what you know. The other person wants to know what he or she should do.  (Even the ball player likes to know you are watching.)
3. When you are friends and you know the suggestion is OK.  However, even then, you should be careful not to seem powerful.
4. When your nation or another group (your job?) really does have power. Then, if you don't want to do what they say, you are deciding not to give them that power of words.

Now let's see an Octopus, and some useful points on the Imperative.