The given pattern is ACT,BDT,CET,DFT…..

We can split the sequence into three:

The sequence of the first alphabets: A, B, C, D, ... 

The next term in the above sequence is E.

The sequence of the second alphabets: C, D, E, F, ..

The next term in the above sequence is G.

The sequence of the third alphabets: T, T, T, T, ..

The third letter in all the terms is T. So, the next term in the above sequence is T.

On combining the terms from the three sequences, the next term of the pattern is EGT.

So, the pattern is ACT, BDT, CET, DFT, EGT.