Breakdown of Puedes sentarte en el escritorio mientras no sea el mío.
ser
to be
no
no, not
en
in / on / at
poder
to be able to / can
Questions & Answers about Puedes sentarte en el escritorio mientras no sea el mío.
Why is sea in the subjunctive here instead of the indicative es?
Because it follows mientras in a conditional sense. When mientras is used to set a condition for an action, it requires the subjunctive. If it were just stating a parallel fact, it would use the indicative.
How do I know if mientras means 'while' or 'as long as'?
You can tell by the verb that follows it! Mientras followed by an indicative verb means 'while' (two things happening at the same time). Mientras followed by a subjunctive verb, like sea in this sentence, changes the meaning to 'as long as' or 'provided that'.
Why do we use en el escritorio instead of al escritorio for 'at the desk'?
Why is it el mío and not just mío?
El mío is a possessive pronoun replacing the noun phrase el escritorio. You need the article el to show it stands in for that specific object ('my desk').