How do I know whether to send an ordinary message or an I message?

If the message you send shows a green background, it is an ordinary message; If the sent SMS is displayed on a blue background, it is an iMessage message. In addition, the small print of "SMS/MMS" will appear above a group of ordinary short messages, and the small print of "iMessage" will appear above a group of iMessage messages. If you pay attention to observation, the difference between the two is still obvious.

For reference.