Yes — all three sentences are correct and natural. If you had let me know, things could’ve been different. - talking about the past, showing regret. If you weren’t going to show, you could at least let me know. - natural, but even smoother is “you could have at least let me know.” You should have at least let me know you would be late. - perfect and natural. So the only small adjustment is that in the second sentence, most people would say “could have” instead of “could.”

Yes — all three sentences are correct ✅, and they sound natural. Nicely done! Here’s a quick breakdown: 1️⃣ “If you had let me know, things could've been different.” This is 3rd conditional → talking about a past situation that didn’t happen. Meaning: You didn’t tell me, and because of that, the result was worse. 2️⃣ “If you weren't going to show, you could at least let me know.” This is a mixed conditional / informal past reference. More natural to say: “If you weren’t going to show up, you could have at least let me know.” It’s like blaming someone for not sending a message before they decided not to come. 3️⃣ “You should have at least let me know you would be late.” This is perfect modal (“should have + past participle”) → expressing criticism about the past. Meaning: It was your responsibility to tell me you’d be late. 👉 Small improvement for #2: ✅ “If you weren’t going to show up, you could have at least let me know.” ⚡ Bonus practice: Try rewriting each sentence in a softer, more polite way — for example: “I wish you had let me know — it would have helped a lot.” “If you couldn’t come, a quick message would’ve been appreciated.”

If you weren't going to show, you could have at least let me know or you could at least have let me know. The others are good.