Conclusion
The easiest way how to sum up findings of the research is to answer the questions and hypotheses in the introduction by means of listing.
1.What are the differences between American and British English in passive voice containing modal verb constructions? When discussing the presence of passive voice between these two types of English, passive voice occurs in greater amount in British English (77 occurrences). 57 sentences were of American origin.

2.Is passive voice predominant in literature for children or in literature for grownups?
My hypothesis that passive voice is less used in books that are focused mainly on juveniles seems to be incorrect. No differences in the frequency of passive voice usage were found. As it was mentioned before, more passive voice sentences were found in Harry Potter than in Hannibal.

3.Why was passive voice used instead of active?
In the grammatical background, seven situations in which passive voice is suitable were noted: when the agent of the action is unknown, unimportant or obvious; in academic writing; to avoid responsibility; to emphasise the agent; to put long subject at the end of clause or sentence; to put older information at the beginning of clause and when something is reported. In 98 out of 134 cases, the reason of passive voice was unknown, unimportant or generally obvious agent. The emphasis of agent was the second most frequent situation in which passive voice was used. Other types of usage are relatively rare.

4.Which modals are mostly used with passive voice?
Answering this question is easy. The most frequent were central modals. In Harry Potter, 67 sentences with central modal verb were observed. Contrarily, two marginals and only one modal idiom occurred. Situation is similar in Hannibal, where 47 sentences were using central modals, one sentence with modal idiom was found and none of the sentences contained marginal modal. Semi-modals occurred 7 times in Harry Potter and 9 times in Hannibal.

5.What time do passive modal structures refer to most often?
Majority (about 85 %) of central modal referred to present time. In other groups, such extreme difference was not observed or the difference was slight.

6.Are there any disputes between theory and praxis?
No disputes were noticed. Findings are fully compatible with grammatical theory.