From Gerald Erichsen,
Your Guide to Spanish Language.
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Some Spanish adjectives can be used as adverbs — but it is difficult to make generalizations about when you can.

Here are some examples of adverbs in the form of adjectives (known as adverbios adjetivales in Spanish):

• Habla muy claro. He speaks very clearly.
• Anduve rápido, observándolo todo. I walked quickly, observing everything.
• No hay necesidad de empujar fuerte. It isn't necessary to push hard.

Note that in all these cases, it would be possible to replace the adjective-adverb with either a standard -mente adverb or an adverbial phrase: Habla muy claramente. Habla con claridad. Anduve rápidamente. No hay necesidad de empujar con fuerza. Although there are no clear rules on when you can use adjectives as adverbs in this way, a few generalizations can be made:

• Such use of adjectives should be thought of as the exception rather than the rule. Although some adverbs such as claro can be used with a wide variety of verbs, others are restricted to a limited number of verbs or set phrases.
• When an adjective is used as an adverb, the singular masculine form is used.
• Such use of adjectives is more common in Latin America than in Spain.

Adjectives that can be used as adverbs in a wide variety of contexts include alto (high , loud), bajo (low, softly), fuerte (strongly, hard), claro (clearly) and rápido (fast).

Others are used in limited contexts: jugar limpio (to play fair/fairly), ir derecho/directo (to go directly), respirar hondo (to breathe deeply), pensar distinto (to think differently), costar barato/caro (to cost little/much).

Adverbs in a series

If you are using a series of adverbs that end in -mente, the -mente typically is retained on only the final adverb in the series:

• Lo explicó sincera y cariñosamente. He explained it sincerely and lovingly.
• Puede instalar nuestro software fácil, segura y rápidamente. You can install our software easily, safely and quickly.

Note that in these series, the adjective-as-adverb is in the singular feminine form, as it would be if the -mente were retained.