TASK DEFINITION: Given the Question and Answer pair, detect whether the answer is acceptable or not. Denote acceptable answer as "yes", otherwise denote as "no". An acceptable answer is consistent with the context in question, detailed, long, and uses general knowledge of English. Answers that are not definite and suggest a variety of options are not acceptable answers.
PROBLEM: Question: Somehow I am compelled to use the word "metal" to describe strong "intestinal fortitude", and perseverance however I can't find the spelling or any synonym like this. Does a word that sounds like "metal" exist when describing an individual's character? I'm looking for modern or historic usage of the word. 
Answer: Mettle Defined by Oxford Dictionaries as: a person's ability to cope well with difficulties; spirit and resilience. It is commonly seen in the phrase: (put someone) on their mettle meaning, (of a demanding situation) test someone's ability to face difficulties. 'there were regular public meetings where local MPs were put on their mettle and remorselessly pilloried' Source: ODO Other constructions are possible too, for example: test their mettle, apparently an American version (thanks, @PeterCordes) show your mettle, etc. Origins According to Merriam-Webster, mettle was, as @Sparhawk points out in the comments, originally simply a variant spelling of the word metal (which dates to at least the 13th century), and it was used in all of the same senses as its metallic relative. Over time, however, mettle came to be used mainly in figurative senses referring to the quality of someone's character, with a first known usage in 1581. Today, both words can mean "vigor and strength of spirit or temperament," but only metal is used of metallic substances.

SOLUTION: yes

PROBLEM: Question: How can you explain that this double negation is not a double negation? Is there a rule in English about this kind of sentence? PS / Do I have to mention Pink Floyd Copyright ? :-) Edit : Since there are a lot of Pink Floyd related explanation, I'll bring a Freddy Mercury one : "I don't have time for no monkey business", which I also understand as "I don't have time for monkey business". Am I right ? 
Answer: Doubled negatives are often used casually in certain dialects to indicate negative concord, an intensification of negation rather than an inversion of it. This typically happens when both words involved are simple negatives, and is most common with no standing in for a, an, or any alongside don't or ain't. So you can safely assume that He isn't not going to the concert. is double negation proper, because it has emphasis, as is She wasn't unimpressed. because this is litotes, whereas I ain't no hillbilly. is negative concord, because it's obviously casual, and uses ain't no in place of am no or am not a. (It's also a patent lie, but that's beside the point.).

SOLUTION: yes

PROBLEM: Question: I've heard people say "be king" (as in "I can't wait to be king") in movies and TV. Why don't they say "be a king"? Which is correct? 
Answer: Oddly enough, I think "to be king", "to be a king" and "to be the king" are all different. If you say "to be a king", like recognizer says, it would imply one of any number of kings, whereas if you say "to be the king", it would imply one specific king of one specific kingdom. On the other hand, if you say "to be king", it's like you're saying "to have the attributes and/or capabilities that come with being (a/the) king", whichever those are. In other words, one can be "a king" or "the king" and still not "be king", if such states do not apportion to their monarchs the features of "being king". And, one can "be king" even without being "a king" or "the king", if such features are made available in some other way, unlikely as that may be. Note that I'm not talking about metaphorical meanings of "king", here (like, say, the "king of rock'n'roll"), but of literal meanings, such as monarch or ruler.

SOLUTION:
no