The sight of a trail runner in the mountains will prompt surprised, if not confused, stares from other hikers on the path.
It's important to understand that in this sentence, "stares" is not the verb[動詞] "to stare", but the noun [名詞].
stare (複数 stares) : 凝視。
→ The sight {of a trail runner} {in the mountains} will prompt surprised{, if not confused,} stares {from other hikers on the path}.
→ The sight of a trail runner will prompt surprised stares.
→ [The sight of a trail runner]=S [will prompt]=V [surprised stares]=O.
to prompt ~ to cause, to provoke ~ 引き起こす
~ Seeing a trail runner will cause surprised (if not confused) stares (凝視).
<S> will prompt <O>.
→ Not so long ago, <S> would prompt <O>.
→ It wasn't so long ago that <S> would prompt <O>.
→ It wasn't so long ago that the sight of a trail runner would prompt surprised stares.
→ It wasn't so long ago that the sight of a trail runner would prompt surprised stares from other hikers.
<A>, if not <B>, <X>
→ <X> is <A>, and sometimes even <B>
The runner gets surprised, if not confused, stares.
→ The stares are surprised, and sometimes even angry.
Thank you so much🙇♀️
I hope this helps you understand the sentence!