Learn Spanish (Latin America): Ella añadió las gotas a la mezcla rápida y directamente. - She added the drops to the mixture quickly and directly.

In Spanish, when you use two or more adverbs ending in -mente in a row joined by y (and) or o (or), you only attach the -mente ending to the very last one. This stops the sentence from sounding repetitive.

Why is the first word rápida (feminine) instead of rápido?

Adverbs ending in -mente are formed using the feminine singular version of an adjective. When you drop the -mente from the first adverb in a list, it stays in that feminine singular form. Since the adjective is rápido, its feminine form is rápida.

Why use the preterite añadió instead of the imperfect añadía?

The preterite is used here because adding the drops was a single, completed event. If we used the imperfect añadía, it would imply she was in the middle of adding them or that she used to add them repeatedly.

Why isn't there an indirect object pronoun like le before añadió?