Hola Cachoot

In Spanish, the adjective "alerta" remains the same in both singular and plural forms. The sentence "Os sugiero que estéis alerta" is grammatically correct. The word "alerta" is an invariant adjective, meaning it does not change in gender or number. This usage is appropriate when addressing a group (plural 'you') as in this sentence. The suggestion to stay "alerta" (alert or watchful) applies to each member of the group equally, without needing to modify the adjective to "alertas".

Saludos

Silvia