Here Italian differs from English — and from many other languages: verbs of thinking, believing and doubting take the congiuntivo. What you think is presented as your view, not as a fact.

Trigger	Meaning
penso che… / credo che…	I think (that)…
mi sembra che… / mi pare che…	it seems to me (that)…
immagino che… / suppongo che…	I imagine / suppose (that)…
dubito che…	I doubt (that)…
non sono sicuro / sicura che…	I'm not sure (that)…
non credo che…	I don't think (that)…

• Penso che Marcos sia a casa. — I think Marcos is at home.
• Dubito che lui sappia la risposta. — I doubt he knows the answer.
• Mi sembra che faccia caldo. — It seems warm to me.