sache

In a sentence

• Quand je parle, sache que je dis toujours la vérité.

  When I speak, know that I always tell the truth.

  Here, "sache" is the subjunctive present form of the verb "savoir" used in a directive or imperative form to express something the listener should understand or acknowledge.

• Il est important que tu saches que cette tâche est difficile.

  It is important that you know (realize) that this task is difficult.

  In this case, "saches" is the subjunctive present form of "savoir" used after the expression "Il est important que" to reflect necessity or emotion within a subordinate clause.

• Sache bien qu’ils ont fait tout leur possible pour t’aider.

  Know well that they have done all they could to help you.

  Here, "Sache bien" uses "sache" as part of a directive encouraging the listener to firmly understand or be aware of the statement.