He reminded me of what I should otherwise have forgotten.

亏他提醒我, 要不然我就忘。

Don’t go too far afield or you’ll get lost.

别走得太远, 要不然你会迷路的。

Make haste, or (else) you'll be late.

点, 要不然就来不。

Make haste, or we shall miss the train.

一点, 要不然我就赶不上火车。

Prices seem to have stopped rising in the shops, or is that just wishful thinking on my part?

里东西的价格好像不再上涨，要不然就是我的一廂情愿？