The splitting of compound verb forms, including infinitives, is not necessarily an error, but often is awkward.
An infinitive is a verb form containing the word to: to go. It is split when something separates the word to from its partner: to quickly go.
Avoid awkward constructions that would damage the rhythm or meaning of a sentence.
Awkward: She was ordered to immediately leave on an assignment.
Better: She was ordered to leave immediately on an assignment.
Awkward: There stood the wagon that we had early last autumn left by the barn.
Better: There stood the wagon that we had left by the barn early last autumn.
But some constructions require that a compound verb be split. Examples:
The budget was tentatively approved.
I will not concede the election.
How has your health been?
He wanted to really help his mother.
Do not manufacture verbs from nouns. Wrong: She authored the book. Right: She wrote the book.