SAY and TELL

Same meaning but different grammar.

Tell must be followed by a direct object, usually a person

• He told me that I should wait

Say does not have a direct object

• He said (to me) that I should wait

In normal speach, "that" is often not used

• He said I should wait

TALK ABOUT and DISCUSS

Same meaning but Discuss does not need a preposition after it

• We talked about the new strategic plan
• We discussed the new strategic plan

SPEAK and TALK

Same meaning and same grammar.

• Did she speak to you about your expenses?
• Did she talk to you about your expenses?