(2) How can you know if a word is a noun?
Here are a few ways to test a word to see if it is a noun.  

Because the last three tests only work sometimes, it is often best to check your dictionary to check what category the word belongs to, if you have any doubt. However, since dictionaries can not be completely inclusive and up-to-date, you want to be prepared to consider check the possible uses of the word you are thinking about in context with native speakers of the dialect you  want to speak or right.  Remember that it is in general not useful to ask speakers whether the word you are curious about is a noun or not.  Instead use the word in a couple of sentences and ask speakers if a sentence "sounds good" or "makes sense".  Make sure that the speaker knows that you are not looking for reassurance, but information -- you want to know the way to say what you need so it sounds like English to a speaker of English.