Learn Spanish (Latin America): Nunca imaginé lo rápido que pasaría esa época. - I never imagined how fast that time would pass.

You cannot use cómo with an adjective or adverb. In Spanish, the formula lo + [adjective or adverb] + que is the most natural way to express "how [adjective]" in the middle of a statement. It acts as a noun phrase, so you are essentially saying "the fastness that that time would pass."

Doesn't a negative statement like Nunca imaginé usually trigger the subjunctive? Why use the conditional pasaría?

While phrases like no imaginé que often trigger the subjunctive because they express doubt or a lack of reality, this sentence uses an indirect question structure (lo rápido que). You aren't doubting that the time would pass; you are stating a fact about your past realization of how fast it was going to pass. The conditional pasaría translates directly to "would pass," acting as the "future in the past."

Why is the subject esa época placed after the verb pasaría?

In Spanish subordinate clauses (like this one starting with que), it is very common and natural to invert the subject and the verb. While lo rápido que esa época pasaría wouldn't be strictly wrong, keeping the verb pasaría right next to the connector que sounds much more fluid to native speakers.

Why is the preterite imaginé used instead of the imperfect , since imagining is a mental state?