Learn Arabic (Modern Standard): ركضنا حتى لهثنا لهثا.

This is the Cognate Accusative for Emphasis (المفعول المطلق المؤكد للفعل). By repeating the verb's meaning using its verbal noun or masdar in the accusative case (لهثا), it emphasizes that the action truly happened. In English, we often translate this using adverbs like 'really' or 'truly' — hence 'we really panted.'

The lesson mentions using the verbal noun 'without an adjective.' What would happen if we added an adjective after لهثا?

If you add an adjective, like لهثا شديدا (severe panting), it changes the function from emphasizing the reality of the action to describing the type of action. It would mean 'we panted severely' instead of 'we really panted.'

I learned that حتى is often followed by a present tense verb in the subjunctive case. Why is it followed by the past tense verb لهثنا here?

When حتى means 'in order to' or refers to an anticipated outcome, it takes the present subjunctive. However, when it narrates a sequence of events that already happened in the past (meaning 'until [event] actually occurred'), it is followed directly by a past tense verb like لهثنا.

Why does لهثا have an alif at the end?

The word is the verbal noun لهث (lahath) acting as an accusative (mansub) element. Because it is indefinite, it takes the 'an' sound (tanwin fatha). In Arabic spelling, words ending in a tanwin fatha almost always require an added alif at the end, written here as لهثا.