Question

I dispatched the completed manuscript and promptly forgot about it make it a simple sentence

Answer 1

On despatching the completed manuscript, I forgot about it.

Answer 2

The following sentence can be written as a simple sentence in the following way.

'On despatching the completed manuscript, I forgot about it'.

A Simple sentence can be described as a sentence that only one independent clause.

An independent clause is a combination of words which has a verb and a subject and can be treated as a complete thought.

Further, a simple sentence does not have any subordinate clause.