Grammar · Chapter 37
不然 / 否则: or else
advice，不然 / 否则 + what will happen
First the advice, then 不然 (“or else”) and what will happen if it's ignored. 否则 means the same but sounds more formal. No need for a long 如果不…: 快走吧，不然要迟到了 is short and natural.
Examples
- 快走吧，不然要迟到了。 Kuài zǒu ba, bùrán yào chídào le. Let's hurry, or we'll be late.
- 多穿一点儿衣服，不然会感冒。 Duō chuān yìdiǎnr yīfu, bùrán huì gǎnmào. Put on more clothes, or you'll catch a cold.
- 早点睡，否则明天会很累。 Zǎo diǎn shuì, fǒuzé míngtiān huì hěn lèi. Go to bed early, otherwise you'll be tired tomorrow.