Hi Valeriia, the usage of subjunctive or indicative with some verbs depends on the meaning of the verb in the context. If "pensare" is used with the meaning of "to suppose", then you must use the subjunctive: "penso che questa idea sia perfetta"; if it's used with the meaning of "to be convinced", then you can use the indicative: "penso che sei molto bravo" or the subjunctive "penso che tu sia molto bravo". It's a matter of style and linguistic register.

No; this is very bad Italian; children or uneducated people say " penso che questa idea è perfetta " , YOU HAVE TO SAY: "Penso che questa idea SIA perfetta " because after CHE you have to use subsubjunctive mode.

Hello Italkian friend, how have you been? I myself am not an Italian teacher but what I learned about it is that some expressions will take the subjunctive mode. Some of them are as follows: Credo che... suppongo che... Non sono certo che... È necessario che... Non vale la pena che... Immagino che... Non suggerisco che... È probabile che.. Etc. I hope these can help you! Cheers!