Why do you say 'penso che sia...?'

In Italian, the subjunctive (congiuntivo) is used after certain verbs that express:1. an opinion/doubt (penso che, credo che, suppongo che)2. a wish/order (voglio che, insisto che, desidero che)3. Impersonal statements (è bene che, è necessario che)For GCSE, you will mainly need to know the first two, and how to use them. When you use one of those phrases, you have to use a subjuntive verb. You will mainly use the auxiliary verbs: essere and avere. Here is how to conjugate them:                       Essere         Avereio              sia          abbia tu              sia           abbialei/Lei           sia           abbianoi             siamo        abbiamovoi             siate         abbiateloro            siano         abbiano Examples:Penso che abbia perso la mia penna - I think I've lost my penSuppongo che sia vero - I suppose that's true.Credo che l'acqua sia calda - I think the water is hot.