Used with an extreme example to demonstrate a point. = さえ
Not a creature was stirring, not even a mouse.　　生き物は、ネズミさえも動かなくなった

すら and さえ are very close, and in many sentences they are interchangeable, but they are not identical in tone or range.

Both mark an extreme or boundary case and carry the idea of “even.” They present something as the minimum, most basic, or least expected example, implying that if this is true, then other cases certainly are as well.

The difference is mainly in tone and register. すら is more formal, written, and analytical. It often sounds objective, logical, or slightly detached, and it appears frequently in essays, narration, and explanatory prose. さえ is more conversational and emotionally colored. It feels closer to spoken language and often carries a sense of frustration, surprise, or personal reaction. It is very common in everyday speech.

There are also small distributional differences. さえ works more naturally with conditionals (and is covered in a separate grammar entry) and expectations, while すら is more common in factual statements, negations, and written narration.

Take an example:

彼は自分の名前すら書けない。
彼は自分の名前さえ書けない。
彼は自分の名前も書けない。
He can’t even write his own name.

すら frames “his own name” as a logical lower bound. If he cannot do this most basic thing, then nothing more complex is possible. It sounds analytical and is common in written or explanatory contexts.

さえ also marks an extreme case, but with more emotional involvement. It often conveys disbelief, frustration, or shock, and feels more conversational than すら.

も is additive. It treats “his own name” as one more thing on a list of things he cannot do, without highlighting it as an extreme case.

Finally, one of the user notes below says すら is always followed by a negative form. This is not correct.

一番簡単のことも 出きないと言う意味

すら is Always followed by Negative Form afterwards.
Example:
時間をかけても簡単な文章を書くことすら出来ない。