Learn Spanish (Latin America): Debes medir las gotas rápida y precisamente. - You must measure the drops quickly and precisely.

When you have two or more adverbs ending in -mente connected by y (and) or o (or), only the very last one keeps the -mente ending. The earlier ones drop it and just use their feminine adjective form. This prevents the sentence from sounding repetitive, so we say rápida y precisamente instead of rápidamente y precisamente.

Why does the shortened adverb use the feminine form (rápida) instead of the masculine (rápido)?

The suffix -mente comes from a Latin feminine noun meaning "mind." Because of this historical root, all -mente adverbs are built by adding the suffix to the feminine form of the adjective. When you drop the -mente for the first adverb in a list, that feminine form (rápida) is what gets left behind.

Why does rápida keep its written accent mark?

Adverbs ending in -mente (and their shortened forms in a list) keep the exact same written accent mark as their base adjective. Since the adjective rápido has an accent on the first a, both rápidamente and its shortened form rápida keep that accent mark.

Why is medir not conjugated here?