Translation:I drink tea on Sundays.
Has no indication of when I drink tea. Depending on the context, I could be drinking tea next sunday or last sunday
When the extra hanzis are not indicated on the left of 星期天, this implies general time of action. Thus, the plural form is emphasized in the sentence.
It is also possible that the sentence is "I drink tea on Sunday" without the plural form of "Sunday". This refers to the next coming Sunday.
This happens sometimes! You have to go with the answer Duolingo wants you to say. Sometimes re-wordings may lead to confusion of others, so it is better to just go along. My apologies although :(
That's just how things are structured, time is usually one of the first things stated.
Ok, a bit confusing since a previous example with time (in the morning) was used between wǒ and hē （我早上喝茶 if I recall correct）
It isn't wrong. What you wrote should also be accepted. You can either put the subject first: 我星期天喝茶 or you can put the time first 星期天我喝茶. They're both equally right, but just choose to stress different words (either that it is I who drink tea on Sundays, or that it is Sundays that I drink tea).
Why is Sunday -- the time -- placed before the subject? Previously, I've only seen setting descriptors follow the subject.