• 믿습니다 is in "declarative" mode.
To validate it, the English sentence would need a subject (you, he, she, or they). This is obviously not the case.
• Without a subject, the sentence given has to be either in the imperative or propositive mode.
Once you decide on the mode to be used, you then select the style (formal/casual) and the level of politeness (indifferent/respect).
• DLG uses: Imperative mode, formal + respectful style. Hence, 믿으십시오.
But any conjugation in imperative or propositive mode should be acceptable.
I have been learning Korean for the past 5/6 months.
DLG is my first and main source of learning at the moment.
Its lack of feedback especially on grammar questions has forced me to do a lot of search on the internet.
My favourite sites are:
• Morninglands (concise and easy to understand);
• Itaki (Good Youtube videos; great tips);
• Reddit & Quora (background knowledge)
• And of course, the Wiki pages (esp. for verb conjugation.)