Today I'd like to ask you about the "practical" interpretative way of "how". That is, do you really distinguish a use as relative adverb from the one as indirect question?; For in spite of academic, theoretical distinction between these, at the time of practical interpretation (or writing), one doesn't make much of these different aspects. For example.
How he answered her question was impressive.
In this case, the how clause has a sense whose nuance is very subtle and obscure in the way which I introduced to you; This how clause can be understood as either of them, and the meaning doesn't change so much. Besides, this how can be replaced by "the way".
(Considering a magnificent performance of some players) That is how fun Mahjong should be.
(Regarding a draft of some plan)This is how long it takes.
These are more controversial; in these case we cannot directly replace how with the way, because this how entails the nuance of question of degree. But, "X is 'indirect question'" structure looks bizarre at the first glance; "X=indirect question itself" seems a bit strange in epistemological terms. Therefore in these case, it can be inferred that the subjects denote the answer of indirect question, while one of the most authoritative English-Japanese dictionaries which handles grammatical uses very much, says that even these cases are included by "relative adverb" usage of "how".
Thus, as a conclusion, what I'd like you to answer is, what kind use of "how" is this?
OK I must refine my point to ask you about; the grammatical mechanism of how you interpret such sentences as I said above. "X is how Y does Z" structure is simple; in this case "how" is equal to "the way", therefore the sentence is turned into "X is the way (that) Y does Z.", and then it is proved that "X(some behavior, conduct, or the like) is the way(method) of Y doing Z). therefore the formula of "X=the way(method)" is established. What is the problem is, how we make this inferencing system in such sentences as "X is how long it takes". In such cases "how" cannot be replaced by "the way". However, it is also bizarre to connect indirect questions with subject; one might think that we'd better connect the ANSWER of such questions, like "X is the Answer of how long it takes", because it is more natural to relate X to those answers by an equal sign(the relation of "X = Answer, not question itself). Of course I know what this type of sentences stand for, however I beg for technical explanation of grammatical analysis of these.