How does the iPhone know whether it is a text message or a multimedia message?

Don’t type in the subject column. Type in the line below the subject. Leave the subject line empty so that the iPhone will not mistake the text message for a multimedia message.

You need to go to Settings, SMS, turn on "Show Subject Bar", return to the main menu, and go to "SMS". If you click "Send New Text Message" at this time, you will see that there are now two columns where you can write text messages. At the top is the "Topic" column. If you are sending a multimedia message (MMS), write your subject on it. If you want to send just text messages, skip the "Subject" column and write directly below. What you are sending is text messages (SMS).

If you are sending a multimedia message (MMS), you must have content (whether it is text or photos) before you can send it. This is your original problem. When you don't turn on the "topic" display, every time you write a text message, it is treated as the subject of the MMS instead of the text of the SMS.