Hi,
1. Does it mean that using 'in which' still keep the subordinate clause as an adjectival clause? Yes. You can see this better if you change the word order to say, rather awkwardly, they talked about the way which Mary dressed in.
2. Why when we are using 'how', the subordinate clause will become a noun clause? 'Why' is always a hard question to answer. ![Smile [:)]](/emoticons/emotion-1.gif)
Let's consider these two sentences.
Tom knows the answer.
Tom knows how Mary calculated the answer.
The object in the first example is a noun. In the second, it is a noun clause. In other words, a noun clause is something that can be used in place of a noun. It can answer the question 'What does Tom know?'.
3. Does 'how' never appear in an adjectival clause but always appear in the noun clause? I don't like to say 'never', but I can't think of an example of an adjective clause that begins with 'how'.
Best wishes, Clive