Can Latin imperfect be "stopped" in the past or only in the present?
b. With iam diū, iam dūdum, and other expressions of duration of time, the imperfect denotes an action continuing in the past but begun at some previous time (cf. § 466). In this construction the imperfect is rendered by the English pluperfect.
Iam dūdum flēbam. (Ov. M. 3.656) I had been weeping for a long time.
cōpiās quās diū comparābant (Fam. 11.13.5) the forces which they had long been getting ready
To judge by the Ovid example it then followed by present tense (:lacrimas manus inpia nostras ridet et inpellit properantibus aequora remis); while the Cicero example it followed by the perfect tense (subito ad patriae periculum converterunt.).
Specifically, In Cicero there are versions that read compararant (pluperfect) and not comparabant. Is the imperfect valid or natural choice despite being stopped in the past? or the pluperfect has to be preferred? What are other examples?