다 (простой стиль) + 길래 (причина)
the speaker is indicating that it was something that he or she heard from someone else that was the cause or reason for the action stated in the following clauseТо, что говорящий слышал от кого-то (1-я часть), стало причиной, основанием для действия во 2-й части.
같은 반 친구가 한국어 공부가어렵'''다길래''' 요즘 도와주고 있어요.