Question

He was unable to take up the post until early the following year. ( Begin Not until )​

Answer 1

Not until early the following year he was unable to take up the post.

EXPLANATION:  

A complex sentence is a type of sentence that involves two or more clauses one of which is independent and the rest are dependent on the main clause.  

The above sentence has been changed into a complex sentence from a simple sentence with two clauses, the main clause and the subordinate clause. The main clause here is he was unable to take up the post whereas the subordinate clause is not until the following year.

Answer 2

Answer:

not until the following semester he was able to take up the post .

Explanation:

we did not put unable because two negative words are not put in the same sentence .