reduce to
The fire reduced the house to ashes.
把……减少/简化/降级为（某种状态）；使沦为
reduce by
The company reduced its staff by 20%.
减少了（具体数量或百分比）
reduce from ... to ...
The price was reduced from $ 100 to $ 70.
从……减少到……
reduce in size/amount/number
The glacier has reduced in size over the past decade.
在尺寸/数量/数目上减少
be reduced to doing something
After losing his job, he was reduced to begging.
被迫沦落到做某事（常含贬义）
reduce the risk / cost / impact
Wearing a helmet can reduce the risk of head injury.
降低风险 / 成本 / 影响