Imperative - "Indicating authority or command; urgent, necessary" (Webster's). An imperative is a statement of what one must do.
Indicative - "Designating that mood of a verb used to express an act" (Webster's). An indicative explains what is true. It is not a command but expresses the rationale behind the command.
“The great gospel imperatives to holiness are ever rooted in indicatives of grace that are able to sustain the weight of those imperatives. The Apostles do not make the mistake that’s often made in Christian ministry. [For the Apostles] the indicatives are more powerful than the imperatives in gospel preaching. So often in our preaching our indicatives are not strong enough, great enough, holy enough, or gracious enough to sustain the power of the imperatives. And so our teaching on holiness becomes a whip or a rod to beat our people’s backs because we’ve looked at the New Testament and that’s all we ourselves have seen. We’ve seen our own failure and we’ve seen the imperatives to holiness and we’ve lost sight of the great indicatives of the gospel that sustain those imperatives. Woven into the warp and woof of the New Testament’s exposition of what it means for us to be holy is the great groundwork that the self-existent, thrice holy, triune God has — in Himself, by Himself and for Himself — committed Himself and all three Persons of His being to bringing about the holiness of His own people. This is the Father’s purpose, the Son’s purchase and the Spirit’s ministry.”
- Sinclair Ferguson