There are some verbs that include the preposition de; without the preposition the message implied would be different. Let's notice the difference in meaning with and without the preposition de:

acabar de- to have just done something
Ella acaba de entrar a la casa.
She just entered the house.
Ella acabó con nuestro matrimonio.
She ended our marriage.

alegrarse de- to be happy to
Francisco se alegró de verte.
Francisco was happy to see you.
Francisco alegró la fiesta.
Francisco was the life of the party.

tratar de- to try to
Vera está tratando de decirte lo mucho que te quiere.
Vera is trying to tell you how much she loves you.
Vera está tratando con el abogado.
Vera is dealing with the lawyer.

dejar de- to stop (doing something)
Su hermanito no deja de molestarla.
Her little brother won't stop bothering her.
Su hermanito siempre deja los juguetes tirados en el piso.
Her little brother always leaves the toys on the floor.
Su hermanito deja que ella juegue con sus juguetes.
Her little brother lets her play with his toys.

terminar de- to finish+infinitive
Mi bebé ya terminó de comer.
My baby just finished eating.
Mi bebé se terminó la leche.
My baby finished the milk.

Some verbs must have the preposition "de" in order to make sense.

arrepentirse de (ie, i)- to regret
Yo me arrepiento de haber salido con Enrique.
I regret having gone out with Enrique.

jactarse de- to boast of
Mi primo se jacta de ser el campeón de motocross.
My cousin boasts about being the motocross champion.

acordarse de (ue)- to remember
Yo me recuerdo de lo alegres que pasábamos las vacaciones con mis hermanos.
I remember how much fun we had during the vacations with my siblings.

cansarse de- to grow tired of
Los atletistas se cansaron de entrenar una semana consecutiva.
The athletes got tired of training a consecutive week.

quejarse de- to complain about
Las señoras de la fundación benéfica se quejaron de la falta de apoyo.
The ladies from the charity complained about the lack of support.

olvidarse de- to forget*
Mi empleada se olvidó de traer el pan.
My help forgot to bring the bread.

depender de (ie)- to depend on
Mi esposa depende de la decisión que yo tome.
My wife depends on the decision I make.

parar de- to stop (doing something)
Ellas no pueden parar de bailar.
They can't stop dancing.
Ellas se pararon en la mesa y siguieron bailando.
They stepped on the table and continued dancing.

*Note! The reflexive form is followed by a preposition plus
an infinitive, but the non-reflexive form is followed by an
infinitive without a preposition.