Double consonants have stronger sounds. Ex: ㅂ usually has the B sound, but ㅃ always has the P sound. Search about batchim that you'll get some more tips that I can't explain easly. Besides de stronger sound and the batchim rules, you'll just need to learn how to spell those words. And, well, since you can write english that have a lot of confusing sounds, I'm sure you will get it soon
When you pronounce the k (or p, t, ch) in english a little bit of air escapes from your mouth as well. When you pronounce ㅋ, ㅍ, ㅌ and ㅊ this happens too. With ㅃ, ㅉ, ㄸ and ㄲ there shouldn't escape any air so the sound is still strong but cleaner. This also happens in when a word ends with certain consonants (every consonant except ㅁ ㄴ ㅇ ㄹ) as you may have noticed. ㅅ ㅈ ㅌ ㅊ at the end of a word all sound like an unfinished t sound (because you shouldn't let the air escape from your mouth). It is complicated but if you practice enough you will get the hang of it.
If you look up the McCune-Reischauer romanization system you might get a better understanding of the pronunciation. In duolingo they use the romanization system used by the Korean government, but M&R stays true to the Korean pronunciation (which is why it is used by academics who specialize in Korean Studies)
And if you need more explanation or you have other questions about the Korean language I recommend going to talktomeinkorean.com They have really great Korean lessons that are more like short lectures. They're really good as listening practice too.
I notice that sometimes when a syllable has W in it, it will have the either the U symbol or the O symbol. For example: pwae is written 뭐 which has an O (ㅗ) symbol while mwo is written 뭐 which has a U (ㅜ) symbol in it. Can someone explain why it's like this and when to use one or the other?