Question

This is the house. I built it.
change simple into comples sentence

Answer 1

A simple sentence can be converted into a complex sentence by expanding a word or phrase into a subordinate clause - which can be a noun clause, an adjective clause or an adverb clause. Note that it is usually a noun or a noun equivalent that can be changed into a noun clause.

Answer 2

Answer:

this is the beautiful house that i built

hope it is helpful