“I don’t think so” in Japanese?
So we’ve been covering a lesson that’s all about expressing your thoughts in Japanese with “~to omoimasu”:

-Rao-san wa Senta ni inai to omoimasu;
-Konban ame ga furu to omoimasu;
-Watashi mo sou omoimasu;
-Nohon ni tsuite dou omoimasu ka?

When asking my teacher how to make a negative “I don’t think so”, he said it doesn’t exist in Japanese language. As he explained: if you say it in negative ”Watashi mo sou omowanai“ it would mean you have no thoughts in your head at all.

Is that really true?

Question

“I don’t think so” in Japanese?
So we’ve been covering a lesson that’s all about expressing your thoughts in Japanese with “~to omoimasu”:

-Rao-san wa Senta ni inai to omoimasu;
-Konban ame ga furu to omoimasu;
-Watashi mo sou omoimasu;
-Nohon ni tsuite dou omoimasu ka?
 
When asking my teacher how to make a negative “I don’t think so”, he said it doesn’t exist in Japanese language. As he explained: if you say it in negative ”Watashi mo sou omowanai“ it would mean you have no thoughts in your head at all.

Is that really true?

Answer

No. If you want to say “I don’t think so” in Japanese, you can use 思わない.
思う is the dictionary form and 思わない is negative form.

Answer

My pleasure.

Answer

My teacher is great for the most part! But what I noticed is that he really likes to exaggerate some things to bring more contrast to the language differences. So he would often say Smth like “in Japanese language there’s no such thing as [*smth that is common for our language*]” “In Japan they never say/use [*something that we say/use*]”. 
I always take it with a grain of salt, but this (~to omoimasu) example really sparked my curiosity.

Thank you for your answers!

Answer

(Watashi wa) sou (wa) omowanai
＝ I don't think so.

Answer

It is really a pity that you don't have a good teacher. What he said is NOT true at all. ／In that case, you usually just say " (私は) そう (は) 思わない", which means exactly the same as "I don't think so.".