I think（that）learning English grammar is boring.为正确表达.想必你清楚,不多说了.
有意思的是：I think learning English grammar boring.依然正确.这里learning English grammar这一非谓语动词短语(动名词短语)做宾语,boring为宾补.
这样的用法是和think这个词的用法有关系的,think后可接从句,也可在宾语后接补语.
I think you right.是正确的表达,只不过有更多的人愿意用 I think (that) you are right.
但当think的宾语为动名词短语或不定式短语时,常用接宾补的形式.且充当宾语的不定式短语常用it来替,充当形式宾语.如：
I think learning English grammar boring.= I think it boring to learn English grammar.