Learn Hardcore Korean: woryoireneun saeroun juga sijakdoeeoseo gibuni johayo. - On Monday, a new week starts, so I feel good.

-에는 here is 에 (time/place marker) + 는 (topic/contrast marker). While 월요일에 simply means “on Monday,” 월요일에는 means “as for Mondays” or “on Mondays (in particular),” implying a contrast with other days or highlighting Mondays as the topic.

Why is it 새로운 주가 instead of just 새 주?

Both 새로운 and 새 can function as “new” before a noun, but:

새로운 is the full adjective form, more formal and descriptive.
새 is a shorter, colloquial form. In our sentence, 주 (“week”) takes the subject marker 가, so 새로운 주가 means “the new week” (as the subject). You could say 새 주가 in casual speech, but 새로운 주가 is more standard in writing.

What is happening in 시작되어서 grammatically?

Base verb: 시작되다 (“to be started,” the passive of 시작하다).
Connector: -어서 (often shortened to -서) links clauses to show cause or sequence.
So 시작되어서 literally means “(it) has been started and…,” implying “because the new week starts…” or “once the new week starts…”

What's the difference between 시작되어서 and 시작돼서?

They’re the same structure, but:

시작되어서 is the full, written form.
시작돼서 is a spoken contraction (되어서 → 돼서).
Use 시작돼서 in casual conversation; stick with 시작되어서 for formal or written styles.

Why is there no explicit subject like “I” before 기분이 좋아요?

Korean frequently omits subjects when they’re clear from context. Here, it’s understood that 저는 (“I”) am the one feeling good, so you drop 저는 and simply say 기분이 좋아요 (“(my) mood is good”).

How does the connective -아서/어서 function in this sentence?

The ending -아서/어서 links the first clause to the second, indicating cause or sequential action. In 시작되어서 기분이 좋아요, it means “because the new week starts, I feel good” or “the new week starts and so I feel good.”

How can I make this sentence more informal or more formal?

Informal (dropping polite ending):
월요일에는 새로운 주가 시작돼서 기분이 좋아.
Formal (using -습니다 ending):
월요일에는 새로운 주가 시작되어서 기분이 좋습니다.