"Άθροισμα" means sum, i.e. the result of addition. It is rarely used outside mathematics.
- Το άθροισμα των τετραγώνων των δύο κάθετων πλευρών = The sum of the squares of the two catheti
"Σύνολο" is a widely used word with various meanings, all of them indicating totality, entirety. It is a compound word, from the (ancient) preposition συν (=together, with) and the adjective όλος/όλη/όλο (=all). In mathematics, "σύνολο" means "set".
- Θεωρία συνόλων = Set theory (lit. Theory of sets)
- Δύο σουβλάκια, μία σαλάτα και ένα κιλό κρασί, σύνολο επτά ευρώ = Two souvlaki(a?), one salad and a kilo(gram) of wine, in total seven euros
- Το σύνολο των μελών διαφωνεί με τον πρόεδρο = All of the members disagree with the president
- Τα μέλη στο σύνολό τους διαφωνούν με τον πρόεδρο = The members in their entirety disagree with the president
- Μουσικά σύνολα = Musical ensembles
- Η κυρία Α φοράει ένα κομψό σύνολο = Mrs A is wearing an elegant ensemble
I would also like to make some comments on your sentences in Greek. If any moderators think these are off-topic, please inform me, though I believe it's actually a good opportunity to say a few things.
First of all, congratulations! You have reached a point where you can write perfectly understood sentences. However, some points here and there "betray" you (Some may also be typos; if this is the case, I'm sorry for that). So, you have written:
ποια είναι η διαφορά ανάμεσα σ'αυτή η λέξη και "σύνολο";
- σ' (σε) requires the accusative, so the whole noun phrase that follows turns into accusative: σ'αυτή τη λέξη
- when "και" (this stands for "ή" as well) links two (or more) noun phrases, these have to be in the same case. This is a strict grammar rule.
- I'm not sure if this is also a rule, but in all instances that come to my mind when noun phrases are linked with "και" or "ή", all these noun phrases either have an article or they don't. So, since "σ'αυτή τη λέξη" has an article, it is wrong to say 'και "σύνολο"'. More natural wordings would be 'ανάμεσα σ'αυτή τη λέξη και το "σύνολο"' or 'ανάμεσα σ'αυτή τη λέξη και τη λέξη "σύνολο"'. This is not true if personal pronouns are used, for example it is correct to say "ανάμεσα σ'αυτή και τη γυναίκα μου".
- To show the previous rule about cases, if you wanted to ask the difference between "άντρας", which you already know, and "άνθρωπος", which is a new word, you could ask 'ποια είναι η διαφορά ανάμεσα σ'αυτή τη λέξη και τον "άνθρωπο";', or 'ανάμεσα σ'αυτή τη λέξη και τη λέξη "άνθρωπος";', or 'η διαφορά μεταξύ άντρα και ανθρώπου;' (both in genitive and without article) or "μεταξύ του άντρα και του ανθρώπου" (both in genitive with article).
Τώρα καταλαβαίνω καλά.
- I guess this comes from "I see" or "I understand" (though I may be wrong). However, in Greek, καταλαβαίνω (I understand, present tense) is used only during the conversation when an explanation is given (Καταλαβαίνω τι μου λες, αλλά... = I understand what you're telling me, but... -which means that the conversation hasn't finished yet), or for feelings (Καταλαβαίνω τον πόνο σου = I understand your pain). When the conversation/explanation has ended (even if this is moments ago), the understanding is considered done, so we usually use the past tense (aorist) of this verb: (Τώρα) Το κατάλαβα, or Δεν το κατάλαβα.
- This "το" (=it) refers to the thing being explained and it feels odd not to include it. In particular, "Τώρα κατάλαβα!" (without το) is a more or less set phrase used when you finally understand something quite confusing, which you had previously misunderstood or had seemed inexplicable (it means something like "Now it finally makes sense!").
- Using "τώρα" with a verb in the past tense is absolutely common, in case it bothers you.
tl;dr: If you used 'ποια είναι η διαφορά ανάμεσα σ'αυτή τη λέξη και τη λέξη "σύνολο";' and "Το κατάλαβα καλά", with the rest of your sentences, you could pass for a native Greek!
teopap, I must express my sincere gratitude for your compliments and your corrections. I myself am a language teacher, and I love it when someone else takes the time to help me learn another language. I have only been studying Greek for a couple months now, but whenever I begin learning a new language I try to practice forming sentences as soon as I can even if I make a few mistakes (I often say that when learning a language most people make the mistake of not being willing to make mistakes- how can one learn if one does not practice?). Anyway, unfortunately, where I live, there are practically no Greek speakers (I have only met two in my life), so your explanations are invaluable and so greatly appreciated. Thank you so much!
Would it also be possible to say Ποια είναι η διαφορά μεταξύ του "άντρας" και του "γυναίκα"; -- treating the words as neuter items since we are treating them as abstract labels rather than words for things?
Or would one have to explicitly say μεταξύ της λέξης "άντρας" και της λέξης "γυναίκα" if one wanted to quote them in their dictionary form?
Also, for what it's worth, the phrase I learned way back when for asking for a difference was Ποια είναι η διαφορά μεταξύ των λέξεων "άντρας" και "γυναίκα"; -- is that also possible or was that not a good way to ask?
Ποια είναι η διαφορά μεταξύ των λέξεων "άντρας" και "γυναίκα"; is imo the best way to ask. There is no danger of misunderstanding at all (for example, it is clear you are not asking for the difference between two particular people, a man and a woman).
As for Ποια είναι η διαφορά μεταξύ του "άντρας" και του "γυναίκα", this is borderline. Right or wrong, I'm not sure. I remember a teacher in elementary school telling us it is wrong, and I don't use it personally, but I have friends who do. I wouldn't recommend it.