"Lúc" and "khi" both mean "when", but they are used slightly differently depending on the sentence structure.
"Khi" is more general and formal, often used with complete actions or clauses.
"Lúc" usually refers to a specific moment in time, and often followed by a noun or phrase.
📘 Example Sentences:
Khi tôi đến, cô ấy đang nấu ăn.
(When I arrived, she was cooking.)
Lúc đó, tôi đang học bài.
(At that time, I was studying.)
Khi: when, more formal or general, a full clause
Lúc: when / at the moment, more specific, a time or short phrase
In many cases, you can use both lúc and khi, but khi is safer in writing, while lúc is more conversational.
Try to notice how native speakers use them in daily conversations.
📝 Tip: If you’re talking about a specific time (like "at that moment"), use "lúc". If it’s more about an action or event, go with "khi".