と and と思う: words and thoughts
Quotative と marks the content of speech or thought. Track the subject and time inside that content separately from the person speaking or thinking in the main sentence.
Quote what someone said
友達は「明日行きます」と言いました。 reports a friend saying, 'I will go tomorrow.' Distinguishing the quotation from 言いました shows that this reports speech, not proof that the friend actually went.
Direct quotations can preserve the original です or ます. 明日 inside a quotation may refer to the day after the original utterance, so do not automatically calculate it from the present day.
Plain forms before と思う
この本は役に立つと思います。 means 'I think this book will be useful.' The thought's content, 役に立つ, is plain, while 思います makes the overall sentence polite.
Non-past affirmative nouns and な-adjectives include だ: 学生だと思います and 便利だと思います. An い-adjective gives 高いと思います. Put the appropriate plain form for the word class before と.
The position of negation
来ないと思います means 'I think they will not come.' 来るとは思いません means 'I do not think they will come.' The conclusions can be similar, but notice whether the negative is inside the content or applied to the thinking.
When describing another person's thoughts, adding the basis, such as そう言っていました ('That is what they said'), can reduce misunderstanding. と思う presents a judgment; it does not turn that judgment into a verified fact.
Check: Which is the basic correct pattern: 便利と思います or 便利だと思います?
Show the answer and why
Use 便利だと思います. Attach quotative と to the な-adjective's plain affirmative non-past form, 便利だ.