There are a few things that don't follow the standard pronounciation rules.
We'll go over 2 of the most common exceptions.
You'll notice that this doesn't follow the 4-box rule. Instead, there's just three letters stacked. It's really simple to read this: just read from top to bottom.
옴 = ㅇ + ㅗ + ㅁ = nothing + oh + m
One thing to note here is that ㅇ still makes the "ng" sound when it's in the bottom, and no sound when it's in the top: this can be confusing since we haven't seen this format before.
ㅃ isn't a letter we've seen before, but you might notice it just looks like two ㅂ's smushed together. That's because it is!
There's 4 of these: ㅃ, ㅉ, ㄸ, and ㄲ.
These don't really have a corresponding sound in English - instead, they're best described as just sharper versions of their single roots.
For example, ㅃ is less like the b in "bear" but more like the b in the "bap"s you hear in older songs.