Formal:
* Écrivez-le (pronounced eh-cree-vay-luh) - This is the most formal way to say "write it."
* Veuillez l'écrire (pronounced vuh-lay lee-kreer) - This means "Please write it."
Informal:
* Écris-le (pronounced eh-cree-luh) - This is the informal singular form of "write it."
* Écrivez-la (pronounced eh-cree-vay-lah) - This is the informal singular form of "write it," used for feminine nouns.
* Écrivez-les (pronounced eh-cree-vay-leh) - This is the informal plural form of "write it," used for plural nouns.
Examples:
* Écrivez-le sur le tableau noir. (Write it on the blackboard.)
* Veuillez l'écrire dans votre cahier. (Please write it in your notebook.)
* Écris-le pour moi. (Write it for me.)
The best way to choose the correct phrase depends on the situation and the formality of the conversation.