Saying “I think”: と思います
State an opinion with a plain-form thought inside a polite sentence.
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Put the thought first, then と 思います. English starts with “I think (that) …”; Japanese places と after the thought and finishes with “think.” と marks the content of the thought in this pattern.
Use plain forms before と, even when the final 思います is polite. For this pattern, use 学生 だ と 思います, rather than 学生 です と 思います.
The sentence’s final ending controls its overall politeness. A plain form inside the sentence does not make the whole sentence rude. You can keep the thought in the plain form and still address the listener politely with 思います.
An i-adjective goes directly before と: 高い と 思います. In affirmative nonpast statements, nouns and na-adjectives take だ before と: 学生 だ と 思います. Verbs use the appropriate plain form, including negatives such as 来ない.
Examples
けんさん は 学生 だ と 思います。
I think Ken is a student.
明日 雨 だ と 思います。
I think it will rain tomorrow.
この 本 は 面白い と 思います。
I think this book is interesting.
けんさん は 来ない と 思います。
I don’t think Ken will come.