这是一个条件状语从句，主句使用'make sure'强调确保动作的发生，语气坚定且直接。
You don't need to reply instantly every time, but when you think of her in a moment of leisure, make sure to let her know.
I think my boyfriend does an excellent job in this regard. Even when he is busy with work and cannot reply to my WeChat messages promptly, he will always ask me what I'm doing as soon as he has a moment. He often calls me out of the blue to say, "I miss you." If you love her, make sure she feels your affection. When you miss someone, express it openly so they know. Don't use shyness as an excuse. When I like you, I don't care about being faceless; I just want you.
虽然不需要你总是秒回，但在不经意间想起她的时候，请务必记得告诉她。
我觉得我的男朋友在这点上做得非常好。即使他每天忙于工作无法及时回复微信，但只要一有空闲，就会主动问我在做什么。他经常会突然打电话给我，直接说：“我想你。”爱她，就要让她感受到你的心意；想念一个人时，就要大方说出来让对方知道。别拿不好意思当借口，当我喜欢你的时候，我不要脸，我只要你。