Atornillar vs. enroscar
While both atornillar and enroscar can be translated as to screw in English, they are not always interchangeable in Spanish. The difference lies in the specificity of the object and the action. Atornillar is used exclusively for screws, while enroscar describes the general action of twisting any threaded object.
Atornillar
B1This verb specifically means to fasten or secure something using a screw (tornillo). It emphasizes the act of joining objects together with a screw, usually with a tool like a screwdriver or a drill. Think of it as to fasten with a screw.
Voy a atornillar el cuadro nuevo a la pared del salón.
(I am going to screw the new painting to the living room wall.)
El carpintero debe atornillar las bisagras de la puerta con cuidado.
(The carpenter must carefully screw on the door hinges.)
Asegúrate de atornillar bien las patas de la silla para que sea estable.
(Make sure to screw the chair legs on tightly so that it is stable.)
Se me rompió la punta del destornillador mientras intentaba atornillar el soporte.
(The tip of my screwdriver broke while I was trying to screw in the bracket.)
Para montar el mueble, tienes que atornillar los paneles laterales a la base.
(To assemble the furniture, you have to screw the side panels to the base.)
Enroscar
A2This is a broader term that refers to the general action of turning or twisting something so that its thread (rosca) engages with another. It can be used for screws, but it is also the correct verb for bottle caps, jar lids, light bulbs, nuts on bolts, and threaded pipes. It focuses on the rotational motion of engaging threads.
No puedo enroscar la tapa de la botella de agua correctamente.
(I can't screw the cap of the water bottle on correctly.)
Para cambiar la bombilla, primero desenrosca la vieja y luego debes enroscar la nueva.
(To change the light bulb, first unscrew the old one and then you must screw in the new one.)
Ayúdame a enroscar esta tuerca en el perno, está muy dura.
(Help me screw this nut onto the bolt, it's very tight.)
El fontanero tuvo que enroscar dos piezas de tubería para reparar la fuga.
(The plumber had to screw two pieces of pipe together to repair the leak.)
Puedes enroscar el tornillo con los dedos al principio para guiarlo.
(You can screw the screw in with your fingers at first to guide it.)
Summary
In short, use atornillar when you are specifically fastening something with a screw (tornillo). Use enroscar for the general action of twisting any object with a thread (rosca), which includes screws, but also lids, caps, light bulbs, and nuts. Every act of atornillar involves an enroscar motion, but not every act of enroscar is atornillar.