"연구원은 남자가 한국어를 못한다고 추측했어요."
Translation:The researcher guessed that the man couldn't speak Korean.
I can try
연구원은 남자가 한국어를 못한다고 추측했어요
연구원은 = the researcher 남자가 = the man 한국어를 = (in) korean 못한다고 = cannot do (with ㄴ다고 meaning a quote of some sort) 추측했어요 = assumed/guessed
The researcher / assumed (that) / the man / cannot (couldn't) speak korean
The researcher assumed the man cannot speak korean