(Reading) Sound changes.

Certain sounds changed in certain condition. Read the notes below to help you understand more.
섬이(an island) you must be read this as seom-i right? but, actually its read as seo-mi.

Another example,
섬에 (to island), its seo-me, not seom-e.
Why? any words that have this situation where consonants meet with 'ㅇ+vocal' will become consonent+vocal as we read it, because 'o' is actually no sound if it is in the middle of the word.
Another example:
책이 (a book) read as chae-gi, not chaeg-i
한국에 (to korea) read as hanggu-ge, not hangguk-e
낮에 (in the daytime) read as na-je, not naj-e

If the final consonant is 'ㄹ' which is actually and L sound, it will change into R.
일이 (a work) read as i-ri, not il-i
물을 (water) read as mu-reul, not mul-eul

If a 'ㅅ' meet 'ㅂ','ㅅ' and 'ㄱ' it will be sound like this,
못봐요 (cant see) read as mot-ba-yo, not mos-ba-yo
못가요 (cant go) read as mot-ka-yo, not mos-ka-yo
못사요 (cant buy) read as mot-sa-yo, not mos-sa-yo

see? this is the right way to read for these 3 situations. Other than this, you can read as usual.
Happy studying~

Learn how to read in Korean here.