Breakdown of Pienso que tu opinión es interesante.
ser
to be
interesante
interesting
tu
your
pensar
to think / to plan
Questions & Answers about Pienso que tu opinión es interesante.
Why do we use the indicative es here instead of the subjunctive sea?
Because the sentence starts with an affirmative statement of thought: pienso que (I think that). In Spanish, when you state what you believe or think affirmatively, you use the normal indicative mood for the second verb. If the sentence were negative (no pienso que...), it would express doubt, and you would need to use the subjunctive sea.
Why doesn't tu have an accent mark in this sentence?
Here, tu is a possessive adjective meaning "your" (as in, your opinion). The version without the accent mark shows possession. The word tú with an accent mark is the subject pronoun meaning "you" (as in, "you are").
The word opinión is feminine. Why doesn't interesante change to end in "a"?
Adjectives that end in -e in their singular form, like interesante or inteligente, are gender-neutral. They stay exactly the same whether they describe a masculine or feminine noun. You only change them to match number, meaning you would add an -s if the noun were plural (opiniones interesantes).
Could I use creo que instead of pienso que to say "I think that"?