Learn Hardcore Japanese: kinou no ziko no kioku ga mada hakkirisite imasu. - The memory of yesterday’s accident is still clear.

The first の attaches to 昨日 (yesterday) and turns it into a modifier for 事故 (accident). In other words, 昨日の事故 means “yesterday’s accident.” It’s marking a possessive or attributive relationship: “the accident that belongs to (or happened on) yesterday.”

Why is there a second の between 事故 and 記憶?

The second の links 事故 (accident) to 記憶 (memory) in the same way. So 事故の記憶 means “memory of the accident.” When you chain them together—昨日の事故の記憶—it literally reads “yesterday’s-accident’s memory,” i.e. “the memory of yesterday’s accident.”

Why is the particle が used after 記憶 instead of は?

Here が marks 記憶 as the grammatical subject of the verb phrase はっきりしています. The speaker is simply stating that “the memory is still clear.” Using は (the topic marker) could shift the nuance or contrast the memory with something else, but が makes it a straightforward description of the subject.

What nuance does まだ add in this sentence?

まだ means “still” or “yet.” Placed before はっきりして, it indicates that the state of the memory being clear continues up to now. So the sentence emphasizes that the clarity of the memory persists at the present moment.

What is the function of はっきりしている here? Why not just はっきりだ?

はっきりしている comes from the verb はっきりする (“to be clear”). Using the ～ている form expresses a continuing state or condition. If you said はっきりだ, you’d be using だ to attach to a na-adjective (はっきり), which is grammatically possible, but it feels less natural for emphasizing an ongoing state. はっきりしている conveys “it remains clear.”

Could you break down はっきりしている into its components?

Sure:

はっきり – adverbial form meaning “clearly” or as a noun “clarity.”
する – the verb “to do,” turning はっきり into “to be clear” or “to clarify.”
～ている – auxiliary indicating a continuous or resultant state.

So はっきり+して+いる literally means “(it) is in a state of clarity.”

Why is しています in polite form rather than plain している?

The sentence uses しています (the polite form of している) to maintain a polite or neutral tone. In casual speech you could say 昨日の事故の記憶がまだはっきりしている without ます, but the original uses います to be slightly more formal or polite.

How would you translate this sentence into English while keeping those nuances?

A natural translation might be:
“My memory of yesterday’s accident is still very clear.”

Here, “still” corresponds to まだ, “memory of yesterday’s accident” covers 昨日の事故の記憶, and “is … clear” reflects はっきりしています.