Learn Hardcore Japanese: mondai no genin wo siru ni ha, tyokusetu mita hito ni kikuno ga hayai. - To find out the cause of a problem, the fastest way is to ask someone who saw it directly.

V-dictionary form + には means “to do X / for doing X / when it comes to doing X.” Here, 問題の原因を知るには sets the topic/condition: “To know the cause of the problem…” The に marks purpose/occasion, and は topicalizes or contrasts that purpose.

Why is 聞く followed by の in 聞くのが?

の nominalizes the verb phrase, turning 聞く into a noun-like unit so it can be the subject of 早い. The pattern is V-のが + adjective. You might see V-ことが in formal writing, but here 聞くことが早い sounds stiff and less idiomatic; 聞くのが早い is natural.

Does 早い here mean “early” or “fast,” and why not 速い?

It means “quick/efficient” in the sense of “the quickest way.” In such evaluative or figurative uses, 早い is standard. 速い is for physical speed (a fast car, fast runner), so 速い would be odd here.

Why is it 人に聞く and not 人から聞く?

人に聞く = “ask someone” (you initiate the question to that person).
人から聞く = “hear from someone” (you receive the information from them). Both can fit the idea, but the original emphasizes the act of asking. A common variant is 直接見た人から聞くのが早い, which focuses on receiving it from them.

Why is 直接 used without に (i.e., not 直接に)?

直接 functions as an adverb on its own, so 直接見た is perfectly natural. 直接に also exists but is less common and feels a bit heavier; most native speakers prefer plain 直接 in everyday usage.

Why is it 見た (past) instead of 見る?

見た人 means “a person who saw (it),” pointing to an eyewitness of a past event. 見る人 would sound like “a person who (generally) sees,” which doesn’t match the intended eyewitness nuance here.

Who is the subject of 知る? It’s not stated.

It’s an implicit, generic subject: “you/we/one.” Japanese often omits subjects when clear from context. You could make it explicit (e.g., 原因を私たちが知るには), but it’s unnecessary here.

Why is there no object after 見た? What did they see?

Japanese relative clauses commonly omit understood elements. The object is contextually “it” (the incident/problem/scene). You could say 直接問題を見た人 or 直接現場を見た人 if you want to spell it out.

Can I say 知るためには instead of 知るには?

Yes. V-ためには is a more explicit, slightly more formal “in order to.” 知るには is a concise, everyday shorthand with the same meaning.

Can I use は instead of が in 聞くのが早い?

Both are grammatical, but the nuance changes. 聞くのが早い neutrally marks “asking” as the subject of the predicate. 聞くのは早い topicalizes or sets up contrast (e.g., 聞くのは早いが、難しい).

Why use 聞く instead of 尋ねる?

聞く is the default everyday verb for both “ask” and “listen/hear” and collocates naturally with 人に. 尋ねる is a more formal “inquire,” fine in careful writing. Polite/humble options include 伺う and お聞きする (e.g., 直接見た人にお聞きするのが早いです).

What exactly is being judged by 早い here?

It evaluates the method as “quick/efficient.” Paraphrase: “The quickest way (to find out) is to ask someone who saw it directly.”

Is 直接見た人から聞くのが早い also natural?

Yes. It means “It’s quickest to hear it from someone who saw it directly.” This slightly shifts the viewpoint from the act of asking (に聞く) to the source from whom you get the information (から聞く). Both are common.

Can I add 一番 for emphasis?

Yes: 直接見た人に聞くのが一番早い is very natural and commonly said to mean “the fastest/best way is to ask an eyewitness.”

Why are there spaces between the words? Do Japanese sentences normally use them?

Standard Japanese writing does not use spaces; they’re added in learner materials to show word boundaries. The normal rendering is 問題の原因を知るには、直接見た人に聞くのが早い。

Why 問題の原因 and not 問題が原因?

Xの原因 means “the cause of X.” Xが原因 means “X is the cause (of something else).” Here we want “the cause of the problem,” so 問題の原因 is correct. Example: 天候が原因で遅れた = “We were delayed due to the weather.”

Could I use synonyms like 手っ取り早い?

Yes. 手っ取り早い strongly emphasizes speed/ease: …に聞くのが手っ取り早い. Other options depend on nuance, e.g., 確実だ (reliable), 有効だ (effective), or 近道だ/早道だ (shortcut), though 早道 is less common in conversation.