Here's a breakdown:
"Say" is used:
* In the present tense:
* *They say it's going to rain tomorrow.*
* *I say we should order pizza.*
* As an imperative:
* *Say your name.*
* *Say it again.*
* In the simple past tense for general statements or habitual actions:
* *He always says he's going to exercise more.*
* *They used to say that the world was flat.*
"Says" is used:
* In the present tense with a singular subject:
* *The sign says "No Parking."*
* *My friend says she's coming.*
* In the simple past tense with a singular subject:
* *The teacher says we have a test tomorrow.*
* *He said he was going to the store.*
* To introduce direct quotes:
* *“I’m going to the beach,” she says.*
* *“I love you,” he says.*
Key difference: "Say" is used when the subject is plural or the verb is in the present tense or imperative. "Says" is used when the subject is singular and the verb is in the present or past tense.
Think of it this way: If you can replace "says" with "said," then you're likely using it correctly.