Use 'go downhill' when something is getting worse, often after a positive period.

Negative Progress

It's informal and best used in casual conversations or storytelling.

Colloquial Usage

Don't use it for minor issues; it's for significant declines in quality or performance.

It literally means moving down a slope or incline, often suggesting speed and lack of control.

Use it when describing objects or people moving downhill without guidance or control.

It often indicates a potentially dangerous or unplanned situation.