There are some expressions that never change in Spanish. Some of them are called adverbial phrases because they relate to adverbs.

Place phrases

allá arriba (up there), aquí abajo (down here), por todas partes (everywhere), a la derecha (to the right), a la izquierda (to the left), fuera de (outside of), a lo lejos (in the distance, far), en el centro de (in the center of)

Time phrases

por la mañana (in the morning), por la tarde (in the afternoon), por la noche (in the evening, at night), pasado mañana (the day after tomorrow), hoy día (these days), dentro de poco / en breve (soon), de aquí en adelante (from now on), de vez en cuando (once in a while), en el futuro (in the future)

Mode phrases

de repente (suddenly), de nuevo (again), a la francesa (without saying goodbye), a lo grande (in style), a regañadientes (unwillingly), a ciegas (blindly), a menudo (often), a cántaros (a lot – rain), a hurtadillas (slyly, secretly)

Quantity phrases

al menos (at least), poco a poco (little by little), solamente (only), al por mayor (wholesale), al por menor (retail), poco más (a little more), poco menos (a little less)

Affirmative phrases

por cierto (by the way), sin duda (no doubt), por supuesto (of course), desde luego (of course), cómo no (by all means), seguramente (definitely), verdaderamente (really)

Negative phrases

de ningún modo (in no way), ni con mucho (by all accounts), ni por asomo (no way, no chance)

Doubt phrases

por si acaso (just in case), por si (if), a lo mejor / probablemente (probably)

Nos vemos prontito.