～と思う【～とおもう】

と思う means "I think": it gives your opinion or your guess. What you think goes before と in the plain form, in any tense, and とおもいます思います is the polite version.

The clause stays plain even when the whole sentence is polite: ×面白いですと思います.

Nouns and な-adjectives need だ

Before と, a noun or a な-adjective takes だ. An い-adjective takes none.

• × 彼は学生と思う → 彼は学生だと思う
• × おいしいだと思う → おいしいと思う

"I don't think"

English puts "not" on "think". Japanese usually puts it on the clause before と思う.

おもしろい面白いとおもわない思わない also exists, but it is a flat rejection of someone else's view: "I don't find it interesting at all".

Someone else's opinion

と思う on its own is your own thought. For what someone else thinks, use とおもっている思っている, which describes their opinion as a state.

• × 田中さんは日本語が難しいと思います → 田中さんは日本語が難しいと思っています

Usage notes

These go beyond saying what you think.

Asking for an opinion

どうおもいます思いますか asks what someone thinks. The answer uses と思います.

"I think I'll"

With a volitional verb, と思う states your plan rather than a guess. That is a separate point, ようと思う.