-Ben은 피아노를 배우고 있다. 

-그 고양이는 이상하게 행동하고 있다. 

-우리는 조사를 했다

-Ben take lessons on the piano

=> Your sentence has a correct grammatical structure, but this has the correct usage/ expression.

Ben is learning the piano.

​

-That’s cat move strange

=> Your sentence has a correct grammatical structure, but this has the correct usage/ expression.​

The cat is acting strangely.​

-We were investigate

=> Your sentence has a correct grammatical structure, but this has the correct usage/ expression.​

We did an investigation.

​Thank you, Jiwon~​