+는/은 is added to the end of a noun to mark it as the topic of the sentence. +ㄴ is added to the end of a verb stem to turn that verb into an adjective. When a verb ends with a consonent at the end of its stem, instead of +ㄴ, we add +은 because it makes it easier to pronounce.
Some words that only add +ㄴ in this situation are 크다 (to be big) and 비싸다 (to be expensive), which become 큰 (big) and 비싼 (expensive). Meanwhile, some words that add +은 are 작다 (to be small) and 좋다 (to be good). The adjective forms of these are 작은 (small) and 좋은 (good).
In addition, if the stem ends with ㄹ, it will be removed for example 달다 becomes 단. And if the verbs are irregular verb ends with ㅂ, it will change to 우 and added with ㄴ for example 쉽다 becomes 쉬은.
There is also a little difference between descriptive verbs (explained above) and action verbs.
When action verbs is modified, it will add 는 after the stem regardless the ending is consonant or vowel. For example 먹다 (eat) becomes 먹는 (the one who eats). 가다 (go) becomes 가는 (the one who goes). And for verb stem ending with ㄹ, just drop it and add 는, eg 날다 becomes 나는. I'm not sure what's the meaning tho.
You can open duolingo in browser, there are notes for each topic. I know them from reading these notes.