一度 / 一时【yídù / yìshí】

一度 and 一时 both sit before the verb and limit what follows to a stretch of time. 一度 looks back on a past period that has ended: "at one point", "for a while". 一时 covers a short moment, often one that catches someone off guard: "for the moment", "on the spur of the moment".

一度: a past chapter

一度 is narrative and written. It often follows 曾, and it implies things are different now.

一时: right now, or in one moment

一时 most often comes before a potential complement – verb + 不 + result, saying something can't be done: 一时想不起来, 一时说不清楚, 一时找不到. It works for the present as well as the past.

With 冲动 "impulse" or 高兴 it names the moment that decided something.

Mixing them up

一度 can't describe a brief moment, and it can't refer to now. 一时 doesn't describe a long-standing status.

• × 我一度想不起来他的名字 → 我一时想不起来他的名字
• × 那家公司一时是全国最大的 → 那家公司一度是全国最大的

一度 also counts occurrences in 一年一度, "once a year".