Breakdown of No creo que el rumor sea una mentira.
ser
to be
no
no, not
una
a, an (feminine)
que
that / than
Questions & Answers about No creo que el rumor sea una mentira.
Why is sea used here instead of es?
The phrase no creo que (I do not believe that) expresses disbelief or doubt. When you state that you do not believe something, Spanish requires the subjunctive mood in the second part of the sentence (after que). That is why we use the subjunctive sea instead of the normal present tense es.
If I changed the sentence to say 'I do believe that the rumor is a lie,' would I still use sea?
No, you would switch back to the normal indicative es: Creo que el rumor es una mentira. In Spanish, affirming what you believe (creo que) does not trigger the subjunctive because you are stating what you consider to be a fact. It is specifically the negative version (no creo que) that creates the doubt needed for the subjunctive.
Why do we use a form of ser (sea) here instead of estar (esté)?
Can I leave out una and just say sea mentira?