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The separate form 'por que' is really easy to remember when you think about question contexts or when you're looking for a reason. Like, when someone asks "por que you didn't come?", you're looking for the reason. Now, if the answer comes with the reason built in, then it becomes one word: "Não vim porque I was sick". The trick is to try replacing it with "what's the reason" or ask a question back - if it works, it's separate.
The 'porquê' with an accent is actually a noun, like when you want to talk about the reason as a concrete thing. "Nunca entendi o porquê of that decision" - there you're treating the reason as an object. Then there's 'por quê' separate with an accent, which is rarer; it shows up at the end of sentences when you're asking a question: "You left early? Por quê?". A ton of people mix them up because you don't really use it like that in practice, the whole internet writes it all together and nobody complains, so it ends up seeming like it doesn't matter.
'Because' is the conjunction (explains the reason): "I didn't go because it was raining." 'Why' is the interrogative or relative pronoun (asks a question or introduces relative clauses): "Why did you leave?" or "I don't know the reason why he left." 'The why' is the noun (the reason, the motive): "Nobody understood the why of the decision."
Do you usually get confused when writing, or is it more when reading that the doubt hits you? The thing is, people tend to stick it all together, and then it becomes a mess.
The explanation you saw above isn't wrong, but you can make it more practical. Forget about the grammar categories for a moment and think of it this way: if you can replace it with "reason" or "motive," it's **because** (all together). "I didn't go because it was raining" = "I didn't go for the reason of it raining." When the sentence is a question or when you take away the question and something's missing - "Why did you leave?" - that's **why** (separated). If it's at the end of a sentence and you can throw in an invisible question mark, like "Why?", then it's **why** (all together, with an accent). And **why** (separated with an accent) is basically the "why" but at the end of the sentence: "Why did you do that?"
The trick is really this: because = reason; why = question in the middle of the sentence; why = question at the end (all together); why = question at the end (separated). It's not rocket science, and once you get the hang of it, it becomes automatic.
The simplest way to get it right is to think about _substitution_: if you can swap it for "reason" or "motive," it's **because** together. "I didn't come because it rained" = "I didn't come for the reason it rained." There you go, it's always like that when you're explaining a reason.
Now, when you need to split it into "for what" or "for which," it's because (sorry for the irony) you're in a context of _question or search_. "For what did you leave?" is a question. "The reason for which you left" is when you're trying to introduce a relative clause. And **why** (with an accent) is the same as "for what," but at the end of the sentence: "I'm not going. Why?" - it's like you have an implicit question at the end. "For what?" with an accent is the question standing alone at the end.
Here's a trick that few people mention: say the sentence out loud and see where your voice pauses. When "because" is a conjunction (explaining a reason), your voice doesn't pause before it - it all sounds flowing, "I didn't comeididn'tcomeididn'tcome because it rained." But when it's "for what" (question or relative), you naturally make a small pause, as if they're two separate ideas: "For what... did you leave?" Practice this for a few days and it becomes automatic.
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