Learn Spanish (Latin America): Acababa de enganchar un atún grande en la popa. - I had just hooked a big tuna at the stern.

The structure acabar de + infinitive expresses having "just" done something. To say "I had just hooked" in the past, you use the imperfect tense (acababa de). If you used the preterite (acabé de), it would change the meaning to a literal "I finished hooking," rather than expressing how recently the event happened.

Since this lesson covers the al + infinitive structure, how would using al enganchar change the meaning?

Acababa de enganchar means the action was already completed right before the current moment in the story ("I had just hooked"). If you used al enganchar un atún grande... ("Upon hooking a big tuna..."), it wouldn't stand alone. It would act as a time marker for another event happening at the exact same moment, such as Al enganchar un atún grande, se rompió la cuerda ("Upon hooking a big tuna, the line broke").

Why is it un atún grande and not un gran atún?

Placing the adjective after the noun (un atún grande) specifically emphasizes physical size—it's a very large fish. If you place it before the noun, it shortens to gran (un gran atún) and takes on a more subjective, qualitative meaning, translating closer to a "great" or "magnificent" tuna.

Is the verb enganchar only used for fishing?