المشددة: ㄲ ㄸ ㅃ ㅆ ㅉ

ㄲ ㄸ ㅃ ㅆ ㅉ are not 'double English letters'. They are a stiffer start in the same square. 불 vs 뿔, 자다 vs 짜다.

You will meet them on food first: 떡볶이, 짜장면, 빨대. If you read 짜 as 자, the clerk still knows — you sound like you skipped week one.

Say 바빠요 out loud. Both ㅃ are tensed. If your cheeks go slack, you said 밥아요, which is not a word anyone wants.