"저는 책을 씁니다."
Translation:I write a book.
It looks like they're mostly interchangeable, but 적다 is more used for when you jot something down with little thought.
Yes, with countable nouns in English you have to use an article when using the singular. (A book, the book, or books; a mountain, the mountain, or mountains, etc.) However, with uncountable nouns and sometimes with place nouns, you should use the singular without the indefinite article (water, not a water; air, not an air; salt, not a salt; etc.).
Question! I hear it often in Korean but, some words are seriously starting to confuse me. Is 니 a n sound or an m? It's not just that one either, the 말 (random usage just for ex); Duolingo mic doesn't accept the m sound, and instead use a b sound. I could have sworn it was m but maybe i just need to practice more? Please help, I is confusion. On a side note I wish the Korean mic practices had a slow down so we can mimic sentence structure better; Korean is hard on the tongue.
What I think you're noticing is that if a syllable ends in ㅂ and it's followed immediately by a ㄴ, the ㅂ changes to get pronounced like ㅁ/m and the ㄴ can become softer in that case as well.
입 sounds like "ib" 니 sounds like "nee" 입니다 sounds like "im-nee-da" or "im-mee-da"