"A pencil is a thing."
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This topic is too big for a comment. There are pages of books and many YouTube videos devoted to trying to explain this. Look up subject marking particles, object marking particles and topic marking particles. This is the sort of grammar that the Duolingo format will never be able to explain.
은 and 는 are topic particles and 이 and 가 are subject particles. Now, the difference between subject and topic is that, a topic is what you are talking about. And a subject is the person who is doing the action. Example#1, "I don't like the new star wars movie." Here, the subject is "I" and the topic is "The new star wars movie" Example#2 "오는은 날씨가 좋아요." Meaning- "Today's weather is nice." Here, 오는 (today) is the topic, that is why we are using 은 with it. And 날씨 (weather) is the subject, that is why we are using 가 with it. I hope it makes sense.