In Korean, this sound shifting takes place under some very well-defined rules. Here's one of the more important rules:

At the beginning of a word (the initial position) or at the end of a word (the final position), this letter: sounds like the k in kiss.

However, when you put it between two vowels (the medial position), it changes to sound like the letter g in guess. (See the details)

This means that , meaning baby, is pronounced a-gi , not a-ki, and , meaning who, is pronounced nu-gu .

When you get to the grammar section that covers verbs, you'll be introduced to another significant sound shift.