You can remember the meaning of 不然 with this easy mnemonic:
A retailer staff worked for a high-end bag shop but the shop was not (不) profiting and sold fakes. She was told to stay quiet, otherwise, she would get fired. However (然), she resigned herself after hearing that.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.