I said "almost" because lucia_mosquito's sentence wasn't grammatically correct. The "hast" needed to come at the end of the sentence, and a comma before "was" was required. With those minor corrections, the sentence conveys the same idea as the original.
Regarding the difference between the German Präteritum (e.g. sagtest) and Perfekt (e.g. gesagt hast), the Perfekt is more common in conversation and informal writing, while the Präteritum is more common in formal writing. There's really no difference in the meaning, though.
Wow, really? I thought Präteritum is more like an unfinished action, and so du sagte can be translated not only as you said but also you were saying same as du sagst can be translated both as you say and you are saying, while du hast gesagt is more finished, like you said or you have said...
Also some phrases about past I only see in Präteritum: Ich dachte, dass... not Ich habe gedacht, ..., Ich hatte keine Zeit, not Ich habe keine Zeit gehabt, while some are really seldom in Präteritum. And some are very common in both but in different situations! Like Ich habe das früher nicht gewusst and Wusstest du, dass...? So I just get quite confused because intuitively I can feel some difference in usage but I can't explain it and don't know any kind of rules, just the feeling...
Aha, yes, you've made some very astute observations! What I stated before is a general guideline, but there are certainly exceptions. Certain verbs like haben tend to be used more often in a particular tense, regardless of the context. Similarly, certain phrases and fixed expressions may also use a single verb form exclusively, and it could sound weird if you used a different form.
I do doubt that the Präteritum is used to indicate unfinished actions, however, as the German language is generally considered to have no continuous/progressive aspect. (Continuous/progressive aspect is how, in English, you indicate an incomplete action, e.g. "you are saying" (present continuous) or "you were saying" (past continuous).) There are, however, colloquial ways of expressing the continuous/progressive aspect in modern German. See for example this explanation from Wikipedia:
The following quote was copied on 2015-07-04:
There is no continuous aspect in standard German. The aspect can be expressed with gerade as in er liest gerade meaning he is reading. Certain regional dialects, such as those of the Rhineland, the Ruhr Area, and Westphalia, form a continuous aspect using the verb sein (to be), the inflected preposition am or beim (at the or on the), and the neuter noun that is formed from an infinitive. For example, ich bin am Lesen, ich bin beim Lesen (literally I am on/at the reading) means I am reading. Known as the rheinische Verlaufsform (roughly Rhinish progressive form), it has become increasingly common in the casual speech of many speakers of standard German, although it is still frowned upon in formal and literary contexts. In Southern Austro-Bavarian, the aspect can be expressed using tun (to do) as an auxiliary with the infinitive of the verb as in er tut lesen for he is reading.
I believe that all you have said about the differences between the Präteritum and the Perfekt are very real. This is a very common thing to say "ah, there's no difference", when obviously there is. Surely the difference in the English tenses does not align with the German ones (why would it?), so that difference that you have in English is obviously not there, but other differences are. And this is all the Indikativ we are talking about. In Konjunktiv there are massive differences.
It's a good question, but I don't think "Das ist nicht, was du vorher sagtest" quite translates to "That is not something you said before."
To my knowledge, the only way to interpret this German sentence is to consider "was du vorher sagtest" to be a relative clause that uses the relative pronoun "was". The Duden has an entry for this meaning of "was", which you can read here (definition 1). It may help to look at the examples given and compare them to the examples given for "was" with the meaning of "etwas" here (definition 1).
So you're right that "was" is sometimes just "etwas" shortened, but that's not the case here. In this sentence, they aren't interchangeable: "Das ist nicht, etwas du vorher sagtest" simply isn't a proper German sentence. While "was" can be used as a relative pronoun, "etwas" cannot.
Similarly in English, "what" can be used as a relative pronoun, but "something" cannot.
This is a little difficult to explain, but I hope this helps.
You have to determine it based on the context. In this case, "Dass ist nicht, was du vorher sagtest" is an illogical sentence, so it must be "das". Just like in English, when people say, "You were there", you know they didn't mean "their" or "they're" because then the sentence wouldn't make sense.
Yes, the difference is critical. Saying that would be like saying "That is not that you said before" in English, which is also wrong. Also, don't forget the comma before "was", as it's also necessary. Finally, be aware that you may also come across this construction: "Das ist nicht das, was du vorher sagtest." The only difference is that extra "das" after "nicht".
This question has already been posed and answered here, but I'll try to provide a more thorough explanation. In the sentence "Das ist nicht, was du vorher sagtest", "was" is a relative pronoun (das Relativ) used to begin a relative clause (der Relativsatz) that further explains the pronoun "das" in the main clause (der Hauptsatz). Relative clauses are always surrounded by commas, unless another punctuation mark would go in the same place (like the period in our example sentence).
Also, since relative clauses are subordinate clauses, the relative clause's verb always comes at the end of the relative clause.
For a rather comprehensive look at relative clauses, see this webpage.