forbidden things?

This week I learned about "the" or no article in names of institutions also modals of obligation.

So, I start telling about article in names. It was a little difficult to me identify where and when we have to use the article. sometimes I put "the" in names which hasn't the article, for example I put "the republic" when it was "republic". But I understood in which case we have to put the article. for institutions like organizations or positions we use "the" for example "The president of Korea" and institutions without article like the names of individual people for example "Princess Isabel". I understood much better this topic.

Also I learned much better modals of obligation. There are many ways to tell somethings that are forbidden or permited. We use "can" to say that something is allowed for example "You can take a sample here", we use "can't and mustn't" to say that something are forbidden, for example "You can't take photos" or "you mustn't swim after eat", we use "must and have to" to say that something is important to do for example "You have tyo do your homework" or "You must be quiet in the library", and finally we use "don't have to" to say that somethis is not necessary to do.

I think that these topics were easy, but in modals of obligations I get confused  when I have to put "can, must and have". But now I recognize when we can use each one in the correct moment.