すら is a formal, literary particle that means 'even'. It stresses that something is so extreme or minimal that even that thing is included or impossible. It often appears in negative sentences or with words like 一人, 一度, 一言 to highlight scarcity or absence. For example, '彼は自分の名前すら書けない' means 'He can't even write his own name.'

ですら is a variant used after nouns that refer to people, especially pronouns like 私, 彼, or when you want a heavier emphasis. It is more common in written language. For instance, '私ですら知らなかった' means 'Even I didn't know.'

Note that すら is quite formal and literary. In everyday conversation, you'd usually use さえ or even も. But すら appears frequently in novels, essays, and formal speeches. Don't use it in casual chats with friends unless you're being dramatic or humorous.