Omoimasu is "to think." "To omoimasu" using this verb is one of the Functional Patterns, which shows the speaker's opinion or guess. It corresponds to the English, "I think," "I guess" or "I believe." This pattern follows the plain form.

Katoo-san wa ashita Oosaka ni iku to omoimasu.

Katoo-san wa ashita Oosaka ni ikanai to omoimasu.

Katoo-san wa kinoo Oosaka ni itta to omoimasu.

Katoo-san wa kinoo Oosaka ni ikanakatta to omoimasu.

I think Mr. Katoo will go to Osaka tomorrow.

I don't think Mr. Katoo will go to Osaka tomorrow.

I think Mr. Katoo went to Osaka yesterday.

I don't think Mr. Katoo went to Osaka yesterday.

The negation, "I don't think" should be normally the nai form of the main verb (iku in this case) + to omoimasu. The usage of to omoimasen making use of the negative form of omoimasu, sounds too strong. Such usage is used to make an objection to the listener.

When this pattern follows the noun or the na-adjective, it should be used with the plain form of desu.

Konsaato wa rai-shuu no do-yoobi da to omoimasu.

Konsaato wa rai-shuu no do-yoobi janai to omoimasu.

Konsaato wa sen-shuu no do-yoobi datta to omoimasu.

Konsaato wa sen-shuu no do-yoobi janakatta to omoimasu.

I think the concert will be next Saturday.

I don't think the concert will be next Saturday.

I think the concert was next Saturday.

I don't think the concert was next Saturday.

When it comes after the i-adjective, it should be used with the plain form of the i-adjective as follows.

Ashita no konsaato wa omoshiroi to omoimasu.

Ashita no konsaato wa omoshirokunai to omoimasu.

Kinoo no konsaato wa omoshirokatta to omoimasu.

Kinoo no konsaato wa omoshirokunakatta to omoimasu.

I think tomorrow's concert will be interesting.

I don't think tomorrow's concert will be interesting.

I think yesterday's concert was interesting.

I don't think yesterday's concert was interesting.

The usage of to omoimasu indicates the speaker is now thinking so. In case the speaker does not think so now, but he had thought so at a certain time in the past, to omoimashita should be used.

Kyoo ame ga furu to omoimashita ga, ii tenki ni narimashita.

I thought it would rain today, but it became fine.

It would have been yesterday or this morning that the speaker thought "it would rain." But now "it is fine," contrary to the speaker's guess.

As mentioned in the previous chapter, the sentence ending with to omoimasu is formal because it ends with -masu form. In casual conversation, the root form, omou replaces omoimasu.

Yooko	:	Ano resutoran ni ikimashoo ka?
Keiko	:	Ano mise wa oishikunai to omou.

Yooko	:	Shall we go to that restaurant?
Keiko	:	I don't think it is good (lit. delicious).

Kanda	:	Suzuki-san wa ima uchi ni iru to omou?
Maeda	:	Uun, inai to omou.

Kanda	:	Do you think Suzuki is at home?
Maeda	:	No, I don't.

"What do you think?" is doo omoimasu ka?, and "I think so, too" is Watashi mo soo omoimasu.

Yooko	:	Hawai to Taiwan to dochira ga minami da to omou?
Keiko	:	Watashi wa Hawai no hoo ga minami da to omou. Junko wa doo omou?
Junko	:	Watashi mo soo omou.

Yooko	:	Which do you think it lies more to the south, Hawaii or Taiwan?
Keiko	:	I think Hawaii lies south. What do you think, Junko?
Junko	:	I think so, too.