A particle meaning “even” used to single out an extreme or surprising example (e.g., “even X”).
こんな簡単な問題さえ間違えるなんて信じられない。
I can't believe you even got such an easy question wrong.
彼は週末さえ仕事をしている。
He even works on weekends.
子どもでさえ知っている話だ。
It's a story even children know.
Used in conditional constructions like “〜さえすれば/〜さえあれば” to mean “if only/so long as (X happens or is present), that is enough.”
健康でさえあれば、他には何もいらない。
As long as I'm healthy, I don't need anything else.
合図さえあれば、すぐに出発できます。
If only you give the signal, we can leave immediately.
君さえ来てくれれば、計画は成功するだろう。
If only you come, the plan will probably succeed.
Can indicate addition or emphasis meaning “besides” or “what's more” (used to add another point or stress an additional fact).
彼は宿題を忘れただけでなく、遅刻さえした。
Not only did he forget his homework, he even came late.
料理は美味しいし、値段も安い。サービスさえ良ければ完璧だ。
The food is tasty and the price is low. If only the service were good, it would be perfect.
彼は英語が話せる。さえ、フランス語も勉強している。
He can speak English. What's more, he's studying French too.