You'll need to read quite a bit on this - too much to put in a forum post. You'll also need to get attuned to where 'the' is used by noticing its use in the reading you do and by noticing how native speakers use it. In most cases, to give you the short explanation, you use 'the' in front of a noun when you know which example of that noun you are talking about and/or you believe your listener knows it as well.
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