"das" vs "dass" - I keep mixing them up in writing

I struggled with this for months until someone taught me this dead simple trick: Try replacing the word with "dieses" (this) or "welches" (which). If the sentence still makes sense, it's **das** (one s). If it doesn't work, it's **dass** (double s). Your examples: - Ich weiss, dass er kommt. → Can you say "Ich weiss, dieses er kommt"? Nope, makes no sense. → dass - Das Buch, das ich gelesen habe. → "Dieses Buch, welches ich gelesen habe" → Works! → das Basically: - **das** = article (the) or demonstrative/relative pronoun (that/which). It always refers to a noun. - **dass** = conjunction that introduces a subordinate clause. It doesn't refer to anything, it just connects two clauses. And yes, "dass" replaced the old spelling "daß" after the 1996 spelling reform. You'll see "daß" in older books and some people in Switzerland still use it, but for exams always write "dass".

James's trick with dieses/welches is spot on. I just want to add one more thing that helped me. Think of it this way: dass ALWAYS sends the verb to the end of the clause: - Ich weiss, dass er morgen **kommt**. (verb at the end) - Er sagt, dass er müde **ist**. (verb at the end) If the word before the comma is a noun and the word after refers back to it, that's a relative pronoun = das: - Das Buch, das auf dem Tisch **liegt**. (refers back to Buch) Funny enough, even Germans make this mistake sometimes. I've seen native speakers write "das" when they mean "dass" on social media. So you're not alone haha.