the usage of "also"

The usage of “also” isn’t clear for me. I’ve already noticed that it must generally be put before the word it regards to (i.e. just like sg else), except the case of the subject. Then “also” follows the subject. For. eg. “Peter also went in the house” means that not only Peter did so. But what to do if I would like to use auxiliaries? After reading many English texts, it seemed to me that then “also” must be insert between the auxiliary and the real verb. For e.g. “Peter should also go in the house.” And it seemed to me that it is differently if the subject is a personal pronoun. For e.g. “He also should go in the house” doesn’t means that he should go in, and somebody els should go in, but that he should go in the house and somewhere else, for. e.g. in the stable. However, somebody, an American guy, told me that it is so also in the other case, saying that “the bag must also be yellow” means that here is another rule or condition about the bag. According to him, to say that the bag must be yellow, just like something else, one would say “the bag also must be yellow.” Is it really so? I write something and to be precise, one of my sentence is “the story of […] can also be said to be a very strange one,” meaning that I’ve already mentioned another strange story. I put the part “can also said to be a” into special search, and it didn’t helped be. See: “Language, furthermore, can be also said to be a form of human culture.” Furthermore makes it clear that it is meant that language, just like other things can also said to be[…]” It is so that I’d thought. But really, in another place, a holiday resort is praised with several attributes, and it is added that this place “can be also said to be a paradisal place.” I also tried “also can be said to a”, but it too was used in both ways. I don’t know if it was due to some bad usage. And if either the usage of “subject+aux.+verb” and “subj.+aux.” doesn’t gives a chance to one to express sg in an unambigiuous way, there is any kind of way to do so? With “too,” or “likewise,” or anything. But it should be a simple, and general way. Thank you for your attention.