For the past tense:
* dit - used for he/she/it said (masculine singular)
* dite - used for she/it said (feminine singular)
* dits - used for they said (masculine plural)
* dites - used for they said (feminine plural)
For the present tense:
* dit - used for he/she/it says (masculine singular)
* dit - used for she/it says (feminine singular)
* disent - used for they say (masculine/feminine plural)
For the future tense:
* dira - used for he/she/it will say
* diront - used for they will say
Example:
* "Il a dit bonjour" - He said hello.
* "Elle a dit merci" - She said thank you.
* "Ils ont dit au revoir" - They said goodbye.
Remember that the correct word depends on the gender and number of the subject.