Learn Arabic (Modern Standard): يتكلم الممثل طبيعيا.

The word طبيعيا is made by taking the adjective طبيعي (natural) and adding the "tanwin fath" sound at the end. In Arabic, adding this indefinite accusative ending to an adjective is a common way to turn it into an adverb of manner, much like adding "-ly" in English.

Why does طبيعيا end with an alif (ا) if it is an adverb?

When we add the "tanwin fath" (the "an" sound) to make an adverb, Arabic spelling rules usually require adding a silent alif (ا) at the end of the word to carry that sound. You will see this on almost all adverbs of manner, except those ending in a taa marbuta (ة).

Could we put the subject first and say الممثل يتكلم طبيعيا?

Yes, you can. Starting with the verb (يتكلم) makes it a standard verbal sentence, which is the most common word order in Arabic. Starting with the noun (الممثل) puts slightly more emphasis on the actor, but the overall meaning remains the same.

Where does the word الممثل come from?

It comes from the root م-ث-ل (m-th-l), which relates to representing, resembling, or providing an example. The word الممثل literally means the one who represents or acts.