"The man eats two cakes."

Translation: 남자가 케이크 두 개를 먹어요.

[FurstWasser]

I'm completely boggled when to use "~가" and "~은" or "~는" as a subject particle.

[nleconte]

Check out this thread on reddit

[Calyope3]

... and gets diabetes

[snazzy_cat07]

Why is it 두 개를 and not 둘 or something ......

I AM CONFUSION!

[Gray_Roze]

Before counters, several numbers get shortened, such as 하나 -> 한, 둘 -> 두, 셋 -> 세, and 넷 -> 네.

[goohayoon]

Is there a native korean word for cake instead of 케이크 ?

[Indi.grt]

Why 두 and not 셋?

[nleconte]

셋 is three.

둘 is two.

[omniduo]

So why can't we use 둘 here?

[LiKenun]

Because rules. There's probably an academic paper somewhere explaining the tendency to simplify sounds of commonly used words and expressions, but the most practical explanation is just that it is.

[jeialzt]

Because "counters" exist in Korean. You add these when you are counting certain things in Korean. These are the following counters I know:

개 - for objects (thus used for "two cakes" above)
명 - for people
번 - (used when stating the number of times you did a thing e.g. twice, thrice etc.)

Sadly, Duolingo does not teach detailed grammar concepts like this.