さえ

1. 1Even: marks the most basic or least expected item, implying that everything else is naturally included (often with a negative: “not even ~”).Ex. 1
2. 2Even (someone/something unlikely): highlights an extreme case to show how obvious, easy, or widespread something is.Ex. 2

A neutral pattern used in both conversation and writing to stress an extreme example, frequently in negative sentences such as 〜さえない. すら has almost the same meaning but sounds more literary. The conditional 〜さえ〜ば (“if only / as long as”) is a separate construction built on the same particle.

• (で)すらN1「すら」 is basically interchangeable with 「さえ」 in the 'even' sense, but it is stiffer and more literary, often carrying surprise or displeasure, and sounds awkward in casual speech; however, 「〜さえ〜ば」 ('as long as... then') cannot be replaced with 「すら」.
• 〜さえ〜ばN3「〜さえ〜ば」 must pair with the ば-form to express 'as long as... then' as the sole condition, or with 「のに」 to express regret; 「さえ」 does not pair with ば and instead means 'even,' citing an extreme example or adding an item. They are not interchangeable.
• だにN1「だに」 is close to 「さえ」 only in the 'even' sense, but it has a strong classical flavor and appears mostly in negative set phrases (e.g., 「微動だにしない」); it can usually be replaced by 「さえ」, but not the other way around; it also has the meaning 'just... alone is enough to.'
• はおろかN1「はおろか」 attaches to X and pairs with 「さえ」 or 「も」 in the following clause to form 'not to mention X, even Y,' usually with negative content and a literary tone; 「さえ」 alone cites a single extreme example with a neutral register and can also add an item. The two are often used together but are not interchangeable.
• たりともN1「たりとも」 attaches only to 'one + counter' or 「何人」, and must be followed by a negative or prohibition, expressing that not even the smallest amount is allowed; it is formal and stern in tone. It resembles 「さえ」 only in the 'one + counter + negative' pattern and cannot be substituted elsewhere.
• こそN3「こそ」 is affirmative emphasis meaning 'precisely this,' highlighting the item itself, unlike 「さえ」, which infers other cases from an extreme example; 「てこそ」 means 'only when... then,' which differs in meaning from 「てさえ」. The two are generally not interchangeable.

(で)すら means the same “even” but is more literary, while だに (mostly in set phrases like 夢にだに) and たりとも (“not even one,” with 一 + counter and a negative) are more formal or restricted. はおろか means “let alone,” pairing a lesser case with a more extreme one; こそ singles out one item emphatically (“precisely this”) rather than presenting an extreme example; and 〜さえ〜ば is the conditional “as long as / if only.”