Following Martha's advice I am splitting up a question Compound sentences, the punctuation and mooore.
Let's put what I said / wrote something in the past. And now I want to elaborate some key points of spoken / written. So I start:
What I wanted to say [is/was/are/were] ...
I think the tenses should agree here. So you can have either:
I think generally I'd use slight re-wordings of either one:
Those sound most natural.
These are all pretty equivalent, conversationally. That is, when in the midst of a conversation, if you want to re-word something you just said, you can use any one and the meaning is the same. Think of it this way: you can either say, "What I meant to say just a second ago was ...", where you're re-wording what just happened, or you can say "The thought I'm trying to get across right now is...", meaning the way you expressed yourself just now was not accurately getting across your thoughts, so you're going to try to explain them again. Either way, the eventual meaning is, "I'm going to say something right now to try to better convey a thought I want to express."
If you're referring to a conversation in the past, though, and you're trying to explain that conversation, you should use the past tense versions.
If you wanted to say something different at the time you said it, you should say "What I wanted to say was..."
If you've later reconsidered what you said and now wish you had said it differently, you should say "What I should have said is..."
