i was stuying about  future tense and i found these exampls 

<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-34I-will-become-a-doctor-in-the-future.mp3" style="border: 0px; outline: 0px; font-size: 14px; vertical-align: top; background: rgb(255, 255, 255); margin: 0px; padding: 0px; color: rgb(193, 50, 29); font-family: Helvetica, Arial, sans-serif;">나는 미래에 의사가 될 거야</a> = I will become a doctor in the future 

<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-36Korea-will-become-a-good-country-soon.mp3" style="border: 0px; outline: 0px; font-size: 14px; vertical-align: top; background: rgb(255, 255, 255); margin: 0px; padding: 0px; color: rgb(193, 50, 29); font-family: Helvetica, Arial, sans-serif;">한국이 곧 좋은 나라가 될 것이다</a> = Korea will become a good country soon

<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-37Korea-will-be-a-good-country-soon.mp3" style="border: 0px; outline: 0px; font-size: 14px; vertical-align: top; background: rgb(255, 255, 255); margin: 0px; padding: 0px; color: rgb(193, 50, 29); font-family: Helvetica, Arial, sans-serif;">한국이 곧 좋은 나라일 것이다</a> = Korea will be a good country soon

<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-38This-place-will-become-a-park.mp3" style="border: 0px; outline: 0px; font-size: 14px; vertical-align: top; background: rgb(255, 255, 255); margin: 0px; padding: 0px; color: rgb(193, 50, 29); font-family: Helvetica, Arial, sans-serif;">이 장소는 공원이 될 것이다</a> = This place will become a park
<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-39This-place-will-be-a-park.mp3" style="border: 0px; outline: 0px; font-size: 14px; vertical-align: top; background: rgb(255, 255, 255); margin: 0px; padding: 0px; color: rgb(193, 50, 29); font-family: Helvetica, Arial, sans-serif;">이 장소는 공원일 것이다</a> = This place will be a park
i noticed that whenever “되다” is used i must use  이/가 

in ex n 1 the "<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-34I-will-become-a-doctor-in-the-future.mp3" style="background: rgb(255, 255, 255); color: rgb(193, 50, 29); outline: 0px; border: 0px; font-size: 14px; vertical-align: top; margin: 0px; padding: 0px; font-family: Helvetica, Arial, sans-serif;">는</a>" was used and then "<a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-34I-will-become-a-doctor-in-the-future.mp3" style="background: rgb(255, 255, 255); color: rgb(193, 50, 29); outline: 0px; border: 0px; font-size: 14px; vertical-align: top; margin: 0px; padding: 0px; font-family: Helvetica, Arial, sans-serif;">가</a>"

while in ex 2 in both cases 이/가 were used 

for 이다 i see that in example 3 the "이" was used with subject <a href="https://www.howtostudykorean.com/wp-content/uploads/2015/01/Lesson9-36Korea-will-become-a-good-country-soon.mp3" style="background: rgb(255, 255, 255); color: rgb(193, 50, 29); outline: 0px; border: 0px; font-size: 14px; vertical-align: top; margin: 0px; padding: 0px; font-family: Helvetica, Arial, sans-serif;">한국이 </a>( can we also say 한국은 ) but in example n5 는 was used 

is there some sort of a rule 은/는or i can use 이/가 and 은/는 in these cases equally