YOU SAID:

i could have sworn he was trembling

INTO JAPANESE

私は彼が震えていたと誓うことができた

BACK INTO ENGLISH

I could swear that he was trembling

INTO JAPANESE

彼が震えていると断言できた

BACK INTO ENGLISH

I was able to assert that he was trembling

INTO JAPANESE

私は彼が震えていると断言することができました

BACK INTO ENGLISH

I was able to assert that he was trembling