Learn Italian: Spero che l'ambiente reagisca bene. - I hope the environment reacts well.

Because the sentence starts with Spero che (I hope that). Verbs expressing hope, desire, or emotion force the verb in the second part of the sentence into the subjunctive mood. Since l'ambiente is the subject (an 'it'), we use the third-person singular subjunctive form.

Where does the -isc- in reagisca come from?

Reagire (to react) is one of the many -ire verbs that take an -isc- insert, just like capire or finire. In the present subjunctive, the -isc- is added to all singular forms and the 'they' form. You then add the regular subjunctive endings for -ire verbs (-a, -a, -a, -iamo, -iate, -ano). So, for l'ambiente, it becomes reag-isc-a.

Does the pronunciation of the verb change between the indicative and the subjunctive?

Yes, it undergoes a big pronunciation shift! In the normal present tense, reagisce has a soft 'sh' sound because of the e at the end. But in the subjunctive, the ending is an a, so reagisca takes on a hard 'sk' sound, exactly like the English word 'sky'.

Can I just say Spero l'ambiente reagisca bene, leaving out the ?