réagisse

In a sentence

• Il est important qu'elle réagisse rapidement à la situation.

  It is important that she reacts quickly to the situation.

  The verb 'réagisse' is used in the subjunctive mood because it follows an expression of importance ('Il est important que'). This form indicates a level of uncertainty or situation dependency.

• Je doute qu'il réagisse comme prévu.

  I doubt he reacts as planned.

  Here, 'réagisse' is in the subjunctive mood, triggered by the verb 'douter' which implies doubt or uncertainty regarding the reaction.

• Nous ne savons pas s'ils réagissent ou non.

  We do not know if they react or not.

  In this case, 'réagissent' is in the indicative mood because it refers to a factual statement about their reaction—its form depends on the specific subject 'ils'.