mainly, primarily
El acné es principalmente un problema para los adolescentes.
Acne is primarily a problem for teenagers.
El público consistía principalmente de estudiantes.
The audience consisted mainly of students.
Su enfermedad era principalmente psicológica.
His illness was primarily psychological.
La materia está compuesta principalmente de nada.
Matter is composed mainly of nothing.
El azufre tiene multitud de usos, principalmente industriales.
Sulfur has many uses, mainly industrial.