Why does “alertar” become “alertá-los” in Portuguese?

When an object pronoun attaches to an infinitive, Portuguese does not simply glue it on. The -r falls away, the pronoun turns into -lo/-la/-los/-las, and the newly exposed vowel takes an accent: alertar + os → alertá-los, “to alert them.” The same rule produces fazê-lo (“to do it”), vendê-la, abri-lo. Skip it and the word stops sounding like Portuguese.

The accent depends on the conjugation: -ar verbs take á (comprá-lo), -er verbs take ê (conhecê-la), -ir verbs need none (abri-lo). The same l- form shows up after any word ending in -r, -s or -z — fá-lo from faz. Here it is the dog doing the warning: o cachorro pulou na cama para alertá-los.