Learn Farsi: مدیر به کارمندان اکیداً اخطار داد. - The manager strictly gave a warning to the employees.

The two lines form the Arabic suffix -اً (pronounced '-an'), called tanvin. When added to an Arabic loanword like اکید (strict), it turns the adjective into an adverb of manner, اکیداً (strictly). You will see this suffix a lot in Farsi to show 'how' an action was done.

Wait, the plural ending on کارمندان (employees) sounds like '-an' too. Are these the exact same suffix?

Good catch, they sound identical but are completely different! The -ان on کارمندان is the native Farsi plural suffix used for people and living things. The -اً on اکیداً is the Arabic adverb-making suffix. You can tell them apart in writing because the plural uses the regular letter nun (ن), while the adverb uses the tanvin symbol over an alef (اً).

In English we can just say 'warned the employees'. Why does the Farsi sentence use به (to) instead of the direct object marker را?

This comes down to the compound verb اخطار دادن (to give a warning). Because the manager is literally 'giving a warning TO the employees', the employees act as the indirect object. Therefore, کارمندان takes the preposition به (to) rather than the direct object marker را.

Could I move اکیداً to the beginning of the sentence for emphasis, like we sometimes do in English?

In Farsi, adverbs of manner like strongly prefer to sit right before the verb they modify. Since the verb here is the compound , the adverb goes directly before the whole compound. Moving it to the very beginning of the sentence sounds unnatural in Farsi.