Learn Portuguese (Portugal): O avião é complicado de montar, mas o acabamento é recompensador. - The airplane is complicated to assemble, but the finish is rewarding.

This structure connects a specific subject (o avião) to an evaluative adjective (complicado) and an action (montar). In Portuguese, when the object of the action is placed at the front as the subject, you must link the adjective to the infinitive verb using the preposition de.

If I wanted to say 'It is complicated to assemble the airplane' instead, would I still use 'de'?

No. If you use an impersonal structure where the action itself is the focus, you drop the de. You would say É complicado montar o avião. The de is only required because o avião has been moved to become the specific subject of the sentence.

I know 'montar' means 'to ride' (like a horse). Is it the standard verb for 'to assemble' as well?

Yes, montar is a very versatile verb. While it means 'to ride' in phrases like montar a cavalo, in the context of machinery, models, or furniture, it is the standard word used for 'to assemble' or 'to put together'.

Does 'acabamento' refer only to a physical finish like paint or polish?

It can refer to the physical coating, but it more broadly describes the final result, the finishing touches, or the overall quality of the completed work. It comes directly from the verb acabar (to finish or to end).

If we were talking about multiple airplanes, what parts of the first clause would change?