ils montaient

were going up, were climbing, were rising, were standing guard

→ monter
mont to ride (a bicycle) + aient they imperfect tense
verb · 3rd-person plural, imperfect tense Rare #47,563
were going up, were climbing, were rising The imperfect form of « monter »; the best translation depends on context.
FR: Ils montaient l'escalier en silence.
EN: They were going up the stairs quietly.
were standing guard Used in the expression « monter la garde » (to stand guard).
FR: Ils montaient la garde devant la porte.
EN: They were standing guard in front of the door.
Conjugation of monter
Indicative imperfect
SingularPlural
First montais —
Second montais montiez
Third montait montaient

Et s'ils montaient ?

What if they went up?

— La Plateforme (2019)

Toutes ces grandes phrases qui montaient dans ma gorge.

All those grand phrases rising in my throat.

— Moscou ne croit pas aux larmes (1980)

Les vagues montaient jusqu’à la hauteur du mât de Northstar.

The waves rose as high as Northstar’s mast.

Les tensions montaient.

Tensions were rising.

Ils montaient la garde chaque nuit, ils mettaient des pièges, des filets.

They stood guard every night; they set traps and nets.

— Heureux comme Lazzaro (2018)