What's the difference between σπέρματί σπέρμα σπέρματός

Question

I am looking at the Septuagint and trying to figure out the differences in the word "seed" that God promised Abraham. I thought that the Greek word σπέρμα means "thy seed" in the plural and that the Greek word σπέρματί means “thy seed” in the singular. Thus a correct translation (based on the Septuagint) of Gen. 13:15-16 is this:

This is the Septuagint:
ὅτι πᾶσαν τὴν γῆν, ἣν σὺ ὁρᾷς, σοὶ δώσω αὐτὴν καὶ τῷ σπέρματί σου ἕως τοῦ αἰῶνος. 16καὶ ποιήσω τὸ σπέρμα σου ὡς τὴν ἄμμον τῆς γῆς· εἰ δύναταί τις ἐξαριθμῆσαι τὴν ἄμμον τῆς γῆς, καὶ τὸ σπέρμα σου ἐξαριθμηθήσεται.

With the word "offspring" correctly translated, it would mean this:
Gen.13:15-16 for I will give you and your offspring (singular) forever all the land that you see. 16 I will make your offspring (plural) like the dust of the earth, so that if anyone could count the dust of the earth, then your offspring (plural) could be counted.

Is this correct? Does the Septuagint differentiate singular and plural offspring for Abraham?

Part two of this question is whether Seed in Gen. 3:15 σπέρματός is singular or plural.

Gen. 3:15 καὶ ἔχθραν θήσω ἀνὰ μέσον σου καὶ ἀνὰ μέσον τῆς γυναικὸς καὶ ἀνὰ μέσον τοῦ σπέρματός σου καὶ ἀνὰ μέσον τοῦ σπέρματος αὐτῆς· αὐτός σου τηρήσει κεφαλήν, καὶ σὺ τηρήσεις αὐτοῦ πτέρναν.

From a previous question, I understand that σπέρματός and σπέρματος mean the same word, "Seed". Is it a singular or plural seed?

Thank you for your help.

Answer 1

As noted in the comments, the difference which you observe in Gen 13:15 is one of grammatical case, not number. All of the forms there are singular.* The paradigm looks like this:

Singular:

• σπέρμα (nominative / accusative),
• σπέρματος (genitive)
• σπέρματι (dative)

Plural:

• σπέρματα (nominative/accusative)
• σπερμάτων (genitive)
• σπέρμασιν (dative)

It's like asking in English about the difference between "he", "him", and "his". There is no difference in the referent. The case (nominative, genitive, dative, or accusative) identifies the role of the word in the syntax of the sentence. The three words you highlight are dative (the usual case used for a word in a relationship translated in English using the preposition "to"), accusative (the object case), and nominative (the subject case), respectively.

_{*This is, of course, a collective singular, imitating the Hebrew. The has been previously discussed in relation to the NT reference.}