Cum = When (Imperfect Subjunctive)

Question

North & Hillard Ex 196: the following is to be translated into Latin: "He had almost reached the top of the alps, when some old men came to him in the guise of envoys..."

Ans: " nam ad summas Alpes paene ascenderat cum legatorum more senes quidam eum aggressi profitebantur…"

These certain old men were declaring themselves to be in the character of envoys; but, if cum = when why is third principal part plural (of aggredior), aggressi (they having approached), used instead of imperfect subjunctive, aggrederentur?

Answer 1

Because there can be only one predicate per clause and that is the verb profitēbantur, while aggressī is a participle, describing senēs and standing in for the clause "postquam aggressī sunt": "..when, having approached him, some old men started saying that they were...". To see the effect of two predicates per clause, consider En. "suddenly a man approached asked me". The cum here is inverse:

2.E.4. In inverted clauses, the principal sentence determining the time of the clause, cum ( = quo tempore) having the force of a relative; cum with the indic. always following the principal sentence; never in oblique discourse [..] principal sentence often with jam, vix, vixdum, nondum, tantum quod, and commodum; cum often with subito, repente, sometimes interim, tamen, etiamtum.

It can be reversed as "cum paene ascendisset, senēs eum aggressī profitēbantur". I must say that considering the part about lēgātōrum mōre ("in the guise of envoys") is already expressed, I struggle to imagine what is supposed to follow profitēbantur.