Question

run fast or you will be late .(change into a complex sentence)

Answer 1

you have to run fast or you will be late .

Answer 2

You have to run fast or you will be late is the simple sentences.

A simple sentence can be converted into a complex sentence by expanding a word or phrase into a complex sentence.

This should be a noun clause, adjective clause, or adverb clause.

So, the above sentence is changed according to the simple sentence and converted into complex sentence.