YOU SAID:

the thing htat do is the thing that doesnt exist

INTO JAPANESE

彼がすることは存在しないことだ

BACK INTO ENGLISH

What he does is nonexistent

INTO JAPANESE

彼がやっていることは存在しない

BACK INTO ENGLISH

What he's doing doesn't exist

INTO JAPANESE

彼がやっていることは存在しない

BACK INTO ENGLISH

What he's doing doesn't exist