Penso che è troppo caro carries on being said by people who could recite the paradigm on request. A conjugation table is usually where the congiuntivo is first met, and sia, abbia and venga get learned there as forms, filed away in good order, and never triggered by anything, because which sentences need them is the one thing a table cannot tell you. The mood is not the difficulty. The difficulty is that nothing in the sentence looks like it is asking for one.
The verb in front of che is what asks. Opinion, doubt, hope and feeling take the congiuntivo: penso che sia, credo che abbia ragione, spero che venga, mi sembra che sia tardi, temo che sia troppo tardi. Knowledge and certainty take the indicativo: so che è caro, è vero che è tardi, ho visto che sei arrivato, sono sicuro che viene. Those two families are not a catalogue of every verb in the language, and nobody is offering them as one. They are the verbs you actually put in front of che in a conversation, and this post answers that question and stops, rather than becoming a guide to the congiuntivo, which is a much larger thing and not what you came here to ask.
Penso che sia troppo caro.
So che è troppo caro.
Spero che venga anche Marco.
Penso di avere ragione.
Secondo me è troppo caro.
Penso che sia troppo caro and so che è troppo caro say the same thing about the price. What differs is the claim you are making about yourself: one reports a view, the other reports a fact, and the verb after che takes its shape from which of the two you chose. That is why no amount of staring at the second half of the sentence helps. The second half is obeying the first.
Two of those sentences get out of the choice altogether, and they are worth having. Penso di avere ragione has no che in it, because the thinking and the being right are both yours: with one subject, Italian uses di and an infinitive, and no mood has to be selected. Secondo me è troppo caro moves the opinion out of the verb and into a phrase, and what is left after it is an ordinary indicativo. Neither of those is a way of dodging the grammar. Both are ordinary Italian sentences.
The edge that trips people up
Negating the opinion changes nothing. Non penso che sia un problema keeps the congiuntivo it had, because what decides the mood is the kind of verb pensare is, and a non in front of it does not make it a report on the world. The interesting direction is the other one: negating a certainty opens the congiuntivo where there was none. Sono sicuro che viene has the indicativo, and non sono sicuro che venga does not, because the certainty has been withdrawn and what is left is doubt.
Non penso che sia un problema.
The second thing is that the tense moves with the main verb. Penso che sia troppo caro is about now, penso che sia andato via is about something already done, and pensavo che fosse più vicino puts the thinking itself in the past and takes the congiuntivo back with it. There is a full system behind that, the sequence of tenses, and it is larger than this page. What is worth knowing here is only that the congiuntivo has its own past and its own imperfetto, so moving penso to pensavo moves the other verb too, and pensavo che è is the same mistake one tense further back.
Pensavo che fosse più vicino.
How to hear it
What settles this is the claim the verb in front of che is making. If you are reporting the world (so, è vero, ho visto, ho letto), the verb after che is the ordinary one, and nothing needs to change. If you are reporting what is in your own head (penso, credo, spero, temo, mi sembra), the verb after che takes the congiuntivo, because what follows is not being presented as a fact about the price or the time. It is being presented as yours. And when the opinion leaves the verb and becomes a phrase (secondo me, per me, or an adverb like forse), there is no opinion verb left to ask, so what follows it is the ordinary indicativo.
That framing also tells you when to stop worrying about the mood at all. If your head and the other verb have the same subject, che is not needed: penso di avere ragione, spero di arrivare in tempo, credo di capire. The construction is shorter and it has no mood to pick.
Penso di is the piece to have ready, because it is the sentence that never has to be decided. It covers every case where the thinking and the doing are both yours, which is most of what you say about your own plans and your own opinions. The congiuntivo is for the other half, the one where somebody else is doing something and you are only giving your view of it, and it starts arriving on its own once you notice that this is what che was announcing.
