We have recently learned the following construction;
1. Watashi wa Mousugu Ame ga Furu to Omoimasu. (I think it’s going to rain soon.)
Since the verb “Omou/Omoimasu” is usually translated to “think”, the above sentence 1 seems to express what you are thinking. But to be more exact, this sentence has a function to say your speculation. Actually, today’s sentence construction is identical to the sentence 1.
2. Watashi wa Risaikuru wa Chikyuu o Mamoru to Omoimasu. (I think Recycling saves our planet.)
Although the example 2 also sounds like a speculation, you can regard 2 as an opinion of the speaker. Let us look at some examples that are expressing opinions using “to Omoimasu”.
3. [V-dic] + to Omoimasu.
Watashi wa Kasa wa Kyou Yaku ni Tatsu to Omoimasu. (I think an umbrella will be useful today.)
– Yaku ni Tatsu: Gr1-T verb; can be useful
4. [V-nakereba naranai] + to Omoimasu.
Watashi wa Kodomo wa Tetsudai o Shinakereba naranai to Omoimasu. (I think kids should help with household chores)
– Nakereba naranai/Nakereba narimasen => Refer to A3 S17
5. [I-adj] + to Omoimasu.
Watashi wa Nihongo wa Muzukashii to Omoimasu. (I think Japanese is difficult.)
6. [Na-adj] + to Omoimasu.
Watashi wa Kuruma wa Benrida to Omoimasu. (I think a car is useful.)
7. [Noun] da + to Omoimasu.
Watashi wa Satou-san no Hanashi wa Hontou da to Omoimasu. (I think what we are talking about Sato-san is real.)
Now we have learned that “to Omoimasu.” can express two things; Speculation and Opinion. They are shown in A1 and A2, respectively. However, I think “to Omoimasu” has a common function to both A1 and A2. That is to avoid making definitive statements. Using “to Omoimasu”, you can make your speculation or opinion more moderate. For example, no one can say we are going to have rain tomorrow for 100 percent sure. Rather than to say “Ashita Ame ga Furimasu”, it is better to say “Ashita Ame ga Furu to Omoimasu”. Another example is; you think recycling is good for our earth, but someone may think recycling doesn’t facilitate new production. If you say “Risaikuru wa Chikyuu o Mamorimasu”, such people may yell at you. If you say “Risaikuru wa Chikyuu o Mamoru to Omoimasu”, your opinion would sound more moderate.
This post was written with reference to the exercise A2 on Section21 of “Minna no Nihongo (2nd Edition)” published by “3A Corporation”