Expressing Opinions: 〜と思う (to omou)

To express an opinion or thought, Japanese attaches 'to omou' (think) to the end of a clause. The clause before 'to omou' MUST be in the Plain Form.

This is a key grammar point: you cannot use 'masu' or 'desu' before 'to omou'. You must switch to the plain (dictionary) form first.

The particle 'と' (to) here works like quotation marks. It means: 'I think [this: ...].'

The JLPT tests this pattern from N5 through N4: at N5 you just need Plain Form + と思う (never masu/desu), and remembering that na-adjectives and nouns need だ before と (元気だと思う, not 元気と思う). At N4, questions start probing the difference between と思う for a thought you're having right now and と思っています for an ongoing opinion or plan you've held for a while.