N3
さえ
Even
彼女は親にさえ知らせずに彼氏と結婚をした。
She got married to her boyfriend, without even telling her parents.
Explanation
「さえ」 means 'even'. You name the most basic, extreme or unbelievable example, hinting that if even that is so, the ordinary case is all the more so: 「親にさえ言えない」 (I can't even tell my parents); 「子供でさえ知っている」 (Even a child knows). Particles: 「が」 and 「を」 are simply replaced by 「さえ」, while 「に」, 「で」, 「へ」 and others are used together with it: 「日本でさえ知られている」 (It's known even in Japan).
Mixed up with
「も」 is the plain 'also'. 「さえ」 clearly gives an extreme example, with surprise, emotion or blame. 「すら」 is N1 written style, stiffer.
「でも」 is the common everyday form. 「でさえ」 works in writing and formal speech, and stresses more strongly than 「でも」.
Easily confused withまで・さえ: the difference ›からして・さえ: the difference ›