Learn Spanish (Latin America): El experto ensambla las piezas hábil y rápidamente. - The expert assembles the pieces skillfully and quickly.

In Spanish, when you link two or more adverbs that end in -mente with a conjunction like y (and) or o (or), you must drop the -mente suffix from all of them except the very last one. This rule stops the sentence from sounding repetitive and heavy.

When we drop -mente, the word is supposed to revert to its feminine singular adjective form. Why doesn't hábil end in an 'a' here?

Adjectives ending in consonants like -l (such as hábil, fácil, or útil) have the exact same form for both masculine and feminine. So hábil actually is serving as the feminine singular form here. If you reversed the order of the adverbs, you would clearly see the feminine form on the other word: rápida y hábilmente.

Why do both hábil and rápidamente have written accent marks in this sentence?

Adverbs ending in -mente always keep the written accent of their root adjective. Because rápido has an accent, rápidamente keeps it. As for hábil, because it dropped its suffix, it stands alone as an adjective. It keeps its standard accent mark because it is stressed on the second-to-last syllable and ends in an 'l' (a consonant other than 'n' or 's').

Could I use the verb instead of for putting pieces together?