Hola Nick

Saber is one of the verbs classified as "verbos de percepción mental" in Spanish grammar. Pensar and creer are also this type of verbs. As per the general rules, these verbs need the subjunctive if the main clause is in negative, and the indicative if the main clause is in affirmative, for example:

"Creo que voy a ir al cine." , "No creo que vaya a ir al cine."

However, verb saber, despite being in this classification, it is an exception. 

If we use it in the present tense, e.g "yo sé, tú sabes..." and it is followed by si, dónde, cómo, cuándo, por qué..., we need the indicative, not the subjunctive, even if it is in negative. 

"Yo sé por qué me mentiste.", "Yo no sé por qué me mentiste.", "Ella sabe cuándo llega Luis.", "Ella no sabe cuándo llega Luis.", "Pedro no sabe si vendrá mañana."

Hope this helps,

Inma