Generate the answer for a given question. The question requires common knowledge of English. Make sure that the answer is in detail, long, consistent with the context, and requires general knowledge of English. To be less repetitive, please vary your language for each question. Give a proper explanation to answer, and if necessary, provide the definition of various terms. And conclude your answer if it's long. If you have used other resources (e.g., Merriam Webster's.) to answer the question, include citations in your answer. You don't need to paraphrase the context of your resources, but make sure that your answer is consistent and complete.

I would like to use a noun made of multiple words (like particle board, Mount Everest, or windscreen wiper) in a compound adjective with a hyphen. But I don't know how to hyphenate such a composition. I'm not sure how common this situation is, so here's an example: Machine learning is a single noun made of two words, and I want to use it in a compound adjective with a hyphen. Are either of the following examples correct? I would like to pursue a machine learning-related certification. I would like to pursue a machine-learning-related certification. If not, then what is the correct way to hyphenate compound adjectives like this?.
Bryan Garner, Garner's Modern American Usage (2003) has a useful discussion of this problem in his lengthy coverage of phrasal adjectives: E. The Compound Conundrum. When the first or last element in a phrasal adjective is part of a compound noun, it too needs to be hyphenated: post-cold-war norms, not post-cold war norms. Otherwise, as in that example, cold appears more closely related to post than to war. In your example, machine learning is the compound noun and related is the additional element in the adjective phrase, so Garner would endorse punctuating it as machine-learning-related. The Chicago Manual of Style, fifteenth edition (2003) takes essentially the same position as Garner: 5.92 Phrasal adjectives. A phrasal adjective (also called a compound modifier) is a phrase that functions as a unit to modify a noun. A phrasal adjective follows these basic rules: ... (2) If a compound noun is an element of a phrasal adjective, the entire compound noun must be hyphenated to clarify the relationship among the words {video-game-magazine dispute} {college-football-halftime controversy}. Words Into Type, third edition (1974) emphasizes the practical goal to be achieved by hyphenating (or not hyphenating): Phrases used as attributive adjectives usually require hyphenation to make clear their relation to the noun they modify. [Examples:] the how-to-study area; a life-and-death struggle If such a phrase modifier is hyphenated at all, it should be hyphenated throughout; but no hyphen should be used between the modifier and the noun. Wrong: low milk-and-cream yielding dam Right: low milk-and-cream-yielding dam Wrong: pay-as-you-go-plan Right: pay-as-you-go plan If there is scarcely any possibility of misreading, hyphens need not be used. [Examples:] a story and a half house; a thirty percent increase; yellow pine timber belt All of these (U.S.) style guides agree that when ambiguity as to what is modifying what might ensue from an unpunctuated adjective phrase, it is better to hyphenate the elements of the phrase than to leave the words open. But the goal is greater clarityand if the hyphens don't clarify anything, they have no business being there. Thus far, my answer has focused on the 'machine-learning-related certification' example in your question. Compound modifiers that incorporate proper nouns of two words or more (such as 'Mount Everestbased climbing expedition') follow a different rule in these style guides: leaving the proper name open, and connecting the compound noun to the adjective with an en-dash instead of a hyphen. I won't go back into the reference works to quote the specific guidelines from each book, but they recommend the approach I have just outlined in this paragraph.

I've found this phrase here on a recent BBC report: '...the mare in the video died after suffering a cardiac arrest on a gallops in April 2016.' The same article has: 'A video on social media shows James mount the animal at a gallops while some present can be heard laughing.' (emphasis mine) It's a bit of a sad story, but I want to know what 'a gallops' is. Google Ngrams has some indication that it might be a phrase rather than just a typo, and also that it might be rising in popularity. TheFreeDictionary redirects me to gallop. I know what 'a gallop' is. It's the plural noun ('a gallops') that I have never heard used before. It feels like some kind of horse racing meet. Any formal (or informal) definition that fits this word? Is it short for something?.
A gallops (or sometimes gallop) is a track or ground specially designed for training or exercising horses - see definition 1.3 here. They usually have a special surface, and might consist of a straight or circular track, often with a fence or rails. Note that this is a British English phrase, equivalent to the American English phrase 'training track'.

Is singular correct, or is plural of 'credential' correct for a single username & password pair?.

This isn't a question you can answer by thinking about the grammar - it's just a case of conventional usage. I've previously worked in the tech industry, and we would always say credentials, never credential. That would still be true even if the credentials were a single item like a certificate. [Edit to add: but Samuel has provided a counter-example to this in a comment on the OP.] In wider English, credential is valid - it's used for example in this Wikipedia article - but it's still more common to use the plural (see Oxford Dictionaries for confirmation). For example, I could say that I have presented my credentials to someone if I have shown them my passport, even though I've only shown them one physical item.