Picar vs. trocear
While both picar and trocear can be translated as to chop in English, they describe different types of cuts in Spanish, primarily distinguished by the size and uniformity of the resulting pieces.
Picar
A2The word picar refers to chopping or mincing food into very small, fine, and often uniform pieces. It is the equivalent of to mince or to dice finely. Think of the small pieces needed as a base for a sauce or a garnish.
Para el sofrito, tienes que picar la cebolla y el ajo muy finos.
(For the sofrito, you have to finely chop the onion and garlic.)
Voy a picar un poco de cilantro fresco para los tacos.
(I'm going to chop a little fresh cilantro for the tacos.)
El carnicero puede picar la carne de ternera para las hamburguesas.
(The butcher can mince the beef for the hamburgers.)
Pica unas nueces para añadirlas al yogur.
(Chop some walnuts to add them to the yogurt.)
Trocear
B1The word trocear means to chop food into larger, often irregular pieces or chunks (trozos). It is used when you don't need a fine or precise cut, and the pieces are meant to be a substantial part of the dish.
Primero, trocea el pollo y las verduras para el guiso.
(First, chop the chicken and the vegetables into chunks for the stew.)
Hay que trocear las patatas antes de asarlas en el horno.
(You have to cut the potatoes into pieces before roasting them in the oven.)
Trocea el pan duro para hacer una sopa de ajo.
(Cut the stale bread into chunks to make garlic soup.)
La receta dice que troceemos la fruta para la macedonia.
(The recipe says to chop the fruit into chunks for the fruit salad.)
Summary
In summary, use picar when you mean to chop something finely or mince it, like herbs, garlic, or onions for a sauce. Use trocear when you are cutting something into larger, more substantial chunks, like potatoes for a stew, meat for roasting, or fruit for a salad.