안녕하세요,
Answers for your questions.
#1: 마음 (ma-eum) "mind" vs, 심장 (shim-jang) "heart"
Generally, yes, both mean "heart"; however, same as English, 마음 differs from 심장.
When your "heart" is broken due to breaking up with your girl-friend or boy-friend, in this case, both "mind" and "heart" mean the same.
"My heart is broken" = "my mind got hurt"
However, when you get a "heart-surgery", "heart" and "mind" are not interchangeable.
"mind-surgery"??? "heart-surgery"...
Do you see my point?
#2: 언제나 (eon-je-na) "always/all the time/whenever/every time"
"Whenever I go to Korean restaurant, I eat Bi-bim-bab." 언제나 한국 음식점에 갈 때마다, 나는 비빔밥을 먹는다.
#3: 은(eun) vs, 는(neun).
We called them, "Topic-Marking Particles". It has the same function and the same usage but, when to use each one is different.
(I highly recommend you to give a listen to Absolute Beginner Season 2 Lesson #3). (I made those series myself).
anyway... to simply explain this, I need some examples.
In Korean sentences, subject needs 'Particles' to be attached.
for example,
"I am Korean."
"I" = 나
"Korean' = 한국사람
"am" = 입니다
Therefore, "I am Korean." = 나 + 한국사람 + 입니다 = 나 한국사람 입니다. HOWEVER!!! you need to attach 은/는 to the subject "I" 나.
so, the correct translation of "I am Korean." is 나"는" 한국사람 입니다.
another example,
"This is water."
"This' = 이것
"water" = 물
"is" = 입니다
therefore, "this is water." = 이것 + 물 + 입니다 = 이것 물 입니다. HOWEVER!!! again, you need to attach 은/는 to the subject "this" 이것.
So, the correct translation of "this is water." is 이것"은" 물 입니다.
Then when you use 는 instead of 은? 은 instead of 는?
It depends on the last syllable of the each subject.
"I" 나 (na), which the last sound (syllable) ends in "a" - VOWEL, then you must attach "는" instead of '은" - 나는!
"This" 이것 (i-geot), which the last sound (syllable) ends in "t" - CONSONANT, then you must attach "은" instead of "는" - 이것은!
There are other particles called, "Subject-Marking Particles" 이/가.
Want to know about them? Please give a listen to 'Absolute Beginner Season 2 Lesson #3'.
In Lesson #4, you can learn about "Object-marking Particles" 을/를 as well.
I hope this helps,
cheers,
Tim