1. 'Dure' (subjunctive) is correct, because how long the war will last is unknown...and that will be reflected in the verb. 2. 'Think about us whenever you need to make renovations on your house.' -- 'necesite' is in subjunctive mood because it is unknown whether they will ever need to make renovations...'necesite' carries in it the same meaning as 'whenever you need'. 3. 'He told him/her to be quiet', or 'he told him/her to shut up'. This is the past tense subjunctive mood.

duré la guerra is subjuntive is not real in that momento because we dont know how long is going to be so is suppositions 2.- think in us when you need to fix your house again subjuntive because is not happening is a suppositions 3.- le dijo que se callará yes is usted/ él /ella told him to be quiet.