Insert the if necessary: It's usually safe to walk on . . . sand, but here, when . . . tide is coming in, . . . sand becomes dangerously soft. . . . people have been swallowed up by it.

It's usually safe to walk on sand, but here, when the tide is coming in, the sand  becomes dangerously soft people have been swallowed up by it.

Dịch: Thông thường, đi bộ trên cát là an toàn, nhưng ở đây, khi thủy triều rút, cát trở nên mềm đến mức nguy hiểm khiến mọi người bị nó nuốt chửng.