〜すら・〜ですら
even (the most basic)
Noun (+ particle) + すら · Noun + ですら
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すら means "even", pointing to an extreme or very basic example: 忙しくて、食事をする時間すらない, "I'm so busy I don't even have time to eat".
It's a more written, emphatic version of さえ (N3), and in most sentences the two are interchangeable. After a noun as the subject, ですら is common: 専門家ですら、この問題は難しい, "even experts find this problem hard".
It's most common with negatives: not even the minimum is possible, like 名前すら書けない or 歩くことすらできない. After a verb, turn it into a noun with こと: 聞いたことすらない.
Compare だけ, "only", and まで, "even (that far)", which extends upward instead of pointing to the bare minimum.
Examples
忙しくて、食事をする時間すらない。
I'm so busy I don't even have time to eat.
彼は自分の名前すら書けない。
He can't even write his own name.
専門家ですら、この問題は難しい。
Even experts find this problem hard.
足が痛くて、歩くことすらできない。
My leg hurts so much I can't even walk.
そんな話は聞いたことすらない。
I've never even heard of such a thing.