In certain code language “BFHMT” is written as “AEIOU”. Then what is the code for the word "HIDDEN"?

Let's check if there is some shift in position.

BFHMT = 2 6 8 13 20

AEIOU = 1 5 9 15 21

-1 -1 +1 +2 +1

There is no pattern visible. In the coded form of a given word, we only have vowels. B is given A, F is given E, and H is given I. This looks like some nearest vowel.

Let's check for the other two letters,

13 (M) is between two vowels, 9 (I) and 15(O). 13 is closer to 15. 

20(T) is between two vowels, 15(O) and 21(U). 20 is closer to 21.

So, it is the closest vowel pattern.

HIDDEN = 8 9 4 4 5 14

So, H is close to I; I is itself a vowel; D is closer to E than A, E is itself a vowel, and lastly, N is close to O. 

The closest vowels are IIEEEO.