A n-p-n transistor conducts when

both collector and emitter are negative with respect to the base collector and emitter are positive with respect to the base collector is positive and emitter is negative with respect to the base collector is positive and emitter is at same potential as the base

collector is positive and emitter is negative with respect to the base

When the collector is positive and emitter is negative w.r.t. base it causes the forward biasing for each junction, which causes conduction of current.