"Goodbye and thank you for the fish."
Translation:Tạm biệt và cảm ơn vì những con cá.
The problem is in the English, not the Vietnamese. The translation of both "những con cá " and "con cá" is "the fish" in English. It is unreasonable for Duo to call an English to Vietnamese translation wrong because "những" is not included when there is no indication of plurality in the question.
A. "Does anyone know, why "vì" is used as "for" in this case?" -> I know some people speak that way. However, most Vietnamese people would likely say:
- Cảm ơn VỀ + noun (phrase):
-- Cảm ơn VỀ buổi tiệc -> Thanks for the party
-- Cảm ơn VỀ bức hình/ảnh đẹp ấy -> Thanks for that beautiful picture
- Cảm ơn VÌ + (reduced) clause:
-- Cảm ơn VÌ (bạn) (đã) giúp tôi -> Thanks for helping me
-- Cảm ơn VÌ (bạn) (đã) mời tôi đến bữa tiệc -> Thanks for inviting me to the party
B. 'I thought it's directly translated as "because".' -> "Vì" can be translated as "for", "because", because of", "due to", etc. depending on the context:
- Cảm ơn VÌ (bạn) (đã) giúp tôi -> Thanks FOR helping me
- VÌ anh yêu em. -> BECAUSE I love you.
- Tôi đã đi học trễ VÌ sáng nay trời mưa to -> I went to school late BECAUSE OF the heavy rain this morning.
- Tôi cảm thấy chóng mặt VÌ thiếu ngủ -> I feel dizzy DUE TO lack of sleep.