否则 / 要不然: otherwise
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Advice/condition，否则/要不然 + negative consequence
否则 (written/neutral) and 要不然 / 要不 (spoken) mean "otherwise, or else". The first part gives advice, an order or a condition; the second, what will happen if you don't.
They open the second clause. Often reinforced with 就 or 会: 否则就会迟到。
Examples
快点儿走吧，否则就赶不上火车了。
Kuài diǎnr zǒu ba, fǒuzé jiù gǎn bu shàng huǒchē le.Let's hurry, or we'll miss the train.
你最好提前预约，要不然可能没有位子。
Nǐ zuìhǎo tíqián yùyuē, yàobùrán kěnéng méiyǒu wèizi.You'd better book ahead, otherwise there may be no seats.
文件必须今天交，否则会影响整个项目。
Wénjiàn bìxū jīntiān jiāo, fǒuzé huì yǐngxiǎng zhěnggè xiàngmù.The document must be handed in today, or it'll affect the whole project.
Common mistakes
✗ 否则 + positive result: ✗ 你要努力，否则你会成功。→ ✓ 否则你会失败。
✗ 要不然 in a written report sounds casual → ✓ 否则。