A transitive verb requires a direct object. The woman wrote a letter. The woman kicked a ball.
WHAT did she write? WHAT did she kick?
Sometimes you follow a transitive verb with an indirect object as well. The woman wrote a letter to her sister. The woman kicked the ball to her teammate.
TO WHOM did she write the letter? TO WHOM did she kick the ball?
An intranstive verb doesn't have a direct object, but it can have an indirect object.
The woman complained to me.
TO WHOM did she complain?
If you leave out the "to" it souds like she "complained you" - an illogical and ungrammatical thing to do.
A common learner mistake is with the verb recommend.
WHAT you recommend is that thing you think well of.
Then you say TO WHOM you recommended that thing.
The woman recommended a good restaurant to me.
WHAT did she recommend? A good restaurant.
TO WHOM did she recommend it? To me.
But we often see "He recommended me" -- which is wrong, unless you were the thing that was recomended. He recommended me [for the job] to the hiring manager.
Student or Learner