Plain-Form + と思う as “I think that”
This grammatical construct is used to express that the subject thinks a certain idea; the particle と is used after this idea to mark it as so. In declarative sentences, the subject of the entire sentence (the one doing the thinking) is implied to be the speaker (first-person), so much so that the respective first-person pronoun that would be used is omitted completely from the sentence. On the other hand, in interrogative sentences, the subject of the entire sentence is implied to be the addressee (second-person).
Kanojo wa ima, kaimono shite iru to omoimasu.
(I think that she’s out shopping right now.)
Kono game wa sugoku tanoshii to omou.
(I think this game is super fun.)
*Note the complete lack of 私は (or any other first-person pronoun followed by は) at the beginning of these two sentences due to the subject, you, already being understood.
Daijoubu da to omoimasuka?
(Do you think it’ll be alright?)
When using the negative form of this grammatical construct it is possible to either conjugate the idea into its negative form or more simply conjugate 思う into its Negative-Form. The former has a stronger sense of thought than the latter.
Kono hamburger wa oishikunai to omoimasu.
(I think this hamburger isn’t delicious.)
*Stronger sense of the hamburger being not delicious.
Kono hamburger wa oishii to omoimasen.
(I don’t think this hamburger is delicious.)