Let us first analyze the statements in question, keeping to the term highlighted here:
The population was alert, as there was a risk of flooding.
all looked alert for the risky maneuvers of the pilot.
O alert it has already been given by the fire department, as there are areas that pose a danger to bathers.
In the first statement, we are faced with an adjective, given that it gives a characteristic to the noun “population”. In this sense, let us return to the particularities related to grammatical classes – bearing in mind the possibility of their being subject to inflection or not. As it is an adjective, it agrees with the noun, in this case, in gender and number: alert – female/population – singular.
In the second example, we see that it is an adverb, as the term “alert” indicates the circumstance in which the verb “looked” is found. Starting from this principle, we have that it remains invariable, in view of its real characteristics.
In the third statement, we note that the term in question is characterized as a noun, sometimes denoting “notice”, “communiqué”, therefore, perfectly susceptible to inflection. We could say that "alerts have already been given by the fire department, as there are areas that pose a danger to bathers." Therefore, we would be making the proper agreement between the verb and the noun.
Based on these assumptions, what we can infer is that the inflection or not of the term under study depends solely and exclusively on the context in which it is used.
Thus, we share with one more particularity that guides the linguistic facts.
By Vânia Duarte
Graduated in Letters
Brazil School Team
Grammar - Brazil School
Source: Brazil School - https://brasilescola.uol.com.br/gramatica/o-termo-alerta-alerte-se-sobre-os-pontos-que-demarcam.htm