He reminded me of what I should otherwise have forgotten.

亏他提醒我, 要不然我。

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.

快点, 要不然来不及。

Make haste, or we shall miss the train.

快一点, 要不然我赶不上火车。

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

商店里东西的价格好像不再上涨，要不然是我的一？