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better than I expected, although not something I would say is totally correct.

Communism is a society in which there is no state, and no economic classes. In essence the people are not divided by any artificial construct of society.

The marxist dictatorship of the proletariat is a method of achieving socialism through which communism, but other forms of socialism exist as well. (market socialists, collectivists, etc,)

There are rules in communism? People are allowed to do some things and not others?

it really depends on what you mean by no rules. It is more the ideology of putting no artificial construct above humanity, like law, capital, etc. One of the worst parts of capitalism is alienation from the product of your labor, so one of the more important aspects of communism is having more impact on your personal environment. That does not mean people will not react, however, so you can't go around hurting other people without repercussions of some kind. In fact people will be more free to react.

"rules" aren't really a concept outside of a monopoly of force by a state-like entity, so there isn't really a good explanation of that specifically. It's like asking about the properties of a square when using a circle smh

Could it be said that there are no rules then?

I don't think the concept of rules applies in this situation. The idea carries a lot of assumptions with it that simply don't make sense in this type of society. It's like asking what the radius of a square is imo

So how do people know what is allowed and what is not allowed?

you have the ability to do whatever you want with the tools you have. The question is what the impact will be and if others will try to stop you. There isn't some god telling you what is and is not allowed, it completely depends on the situation.

Are you asking how people would react to your actions? If they would structure it in a specific way?