Influir vs. influenciar
While 'influir' and 'influenciar' both translate to 'to influence' in English, there is a subtle difference in their use and connotation in Spanish. 'Influir' is more commonly used and usually preferred, especially in Spain, while 'influenciar' is popular mostly in Latin-American countries.
Influir
'Influir' in Spanish means to have an effect on someone or something, indirectly or subtly. It usually does not involve direct persuasion or intentional action.
La publicidad puede influir mucho en las decisiones de compra de las personas.
(Advertising can greatly influence people's purchasing decisions.)
La opinion de mis padres influye mucho en mis decisiones.
(My parents' opinion greatly influences my decisions.)
Influenciar
'Influenciar', although less commonly used, has a stronger connotation than 'influir'. It implies not just an effect, but a direct and intentional effort to affect or shape someone else’s views or actions.
El maestro está intentando influenciar las opiniones de sus estudiantes sobre el medio ambiente.
(The teacher is trying to influence his students' opinions on the environment.)
La cultura popular puede influenciar el comportamiento de los jóvenes.
(Popular culture can influence young people's behavior.)
Summary
'Influir' and 'influenciar', being Spanish equivalent of 'to influence', have nuances in their use. 'Influir' is used more often to mean a subtle or indirect effect, while 'influenciar' implies a direct, often intentional act of influencing someone or something. The choice between these two will depend on the context or your audience's dialect.