Δεν ξεχωρίζω αυτή τη λέξη από την άλλη όταν μιλάς τόσο γρήγορα.

Δεν is the normal negation used with verbs in the indicative, so here it simply means not / I don’t.

• Δεν ξεχωρίζω = I don’t distinguish / I can’t tell apart
• You would use μη(ν) instead in different environments, such as commands or subjunctive structures.

So in this sentence, Δεν is exactly what you would expect with a normal statement.

Here ξεχωρίζω means to distinguish, to tell apart, or to make out as different from something else.

In this sentence, it does not mean just separate in a physical sense. It means:

• Δεν ξεχωρίζω αυτή τη λέξη από την άλλη
  = I can’t tell this word apart from the other one

This verb is very commonly used when talking about noticing differences between things, sounds, people, words, etc.

In Greek, when you use a demonstrative like this or that, you usually also keep the article.

So:

• αυτή τη λέξη = this word
• literally something like this the word

That is normal Greek structure.

Compare:

• η λέξη = the word
• αυτή η λέξη = this word (subject form)
• αυτή τη λέξη = this word (object form)

To an English speaker, the extra article can feel unnecessary, but in Greek it is standard.

Both are related forms of the feminine accusative article, but τη is the shortened everyday form of την before many consonants.

So:

• τη λέξη = normal and very common
• την λέξη = also possible in more careful or formal writing, but less usual here

The full form την is especially kept before vowels and certain consonants, but in everyday Modern Greek you will very often see and hear τη.

So αυτή τη λέξη is completely natural.

Because λέξη is the feminine singular noun word, and here it is the direct object of ξεχωρίζω.

The form of λέξη is actually the same in nominative and accusative singular:

• η λέξη = the word
• τη λέξη = the word as object

So the case is shown mostly by the article:

• nominative: η λέξη
• accusative: τη λέξη

That is why the noun itself does not visibly change here.

Because ξεχωρίζω X από Y means distinguish X from Y.

So:

• αυτή τη λέξη = this word
• από την άλλη = from the other one

The preposition από takes the accusative in Modern Greek, so την άλλη is in the accusative form.

This is a very useful pattern:

• ξεχωρίζω το ένα από το άλλο = I tell one apart from the other
• δεν ξεχωρίζω τη φωνή σου από τη δική του = I can’t tell your voice from his

Yes, από την άλλη λέξη would be grammatically fine, but Greek often leaves out a noun when it is obvious from context.

So:

• αυτή τη λέξη από την άλλη
  literally = this word from the other
• understood meaning = this word from the other word / the other one

This kind of omission is very common and natural in Greek, just as in English you can say:

• this one and the other
• I like the red shirt, not the other

So την άλλη here means the other one, with λέξη understood.

Όταν means when, and μιλάς is the 2nd person singular form of μιλάω / μιλώ: you speak / you are speaking.

So:

• όταν μιλάς = when you speak
• more naturally here: when you’re speaking

The ending -άς tells you the subject is you singular, informal.

If the speaker were addressing more than one person, or using the polite plural, it would be:

• όταν μιλάτε

Greek often uses the present tense for general or repeated situations, just like English does.

So this sentence means something like:

• I can’t tell this word from the other when you speak so fast

This is a general statement about what happens whenever that person speaks quickly. It is not limited to one single moment.

So:

• ξεχωρίζω = I distinguish / I can distinguish
• μιλάς = you speak / you are speaking

In English, you might choose either simple present or progressive depending on style, but Greek present tense covers this kind of meaning very naturally.

Τόσο means so or that, and γρήγορα here is an adverb meaning quickly / fast.

So:

• τόσο γρήγορα = so fast / so quickly

A useful point: γρήγορα looks like an adjective form, but here it functions as an adverb.

Compare:

• γρήγορος = fast (masculine adjective)
• γρήγορη = fast (feminine adjective)
• γρήγορο = fast (neuter adjective)
• γρήγορα = fast / quickly as adverb

So in this sentence it modifies μιλάς: you speak so fast.

Yes. This is a very common thing in Greek.

Many adverbs have the same form as the neuter plural of the adjective, so they may look plural to an English learner, but function as adverbs.

Here:

• γρήγορα = quickly / fast

It does not mean multiple fast things here. It is simply describing the manner of speaking.

This pattern is very common:

• καλός = good

• καλά = well

• αργός = slow
• αργά = slowly / late

So τόσο γρήγορα is a normal adverbial phrase.

Yes, Greek word order is fairly flexible, but this version is very natural.

The sentence is:

• Δεν ξεχωρίζω αυτή τη λέξη από την άλλη όταν μιλάς τόσο γρήγορα.

A rough structure is:

• Δεν ξεχωρίζω = I can’t tell apart
• αυτή τη λέξη από την άλλη = this word from the other
• όταν μιλάς τόσο γρήγορα = when you speak so fast

Greek often places the main verb early, then the object phrase, then the time/circumstance clause.

You could move parts around for emphasis, for example:

• Όταν μιλάς τόσο γρήγορα, δεν ξεχωρίζω αυτή τη λέξη από την άλλη.

That would emphasize the when you speak so fast part a bit more.

Yes, that is a very good instinct.

In this sentence, δεν ξεχωρίζω αυτή τη λέξη από την άλλη suggests a problem in distinguishing one word from another, especially because the other person is speaking quickly.

So the nuance is closer to:

• I can’t tell this word apart from the other
• I can’t make out the difference between this word and the other one

It is not exactly the same as:

• δεν καταλαβαίνω = I don’t understand

You might understand the general meaning of what someone says but still not be able to distinguish one word from another. That is the idea here.

The negative particle can appear as δε or δεν. In standard modern usage, the final -ν is often kept before vowels and certain consonants.

Since the next word is ξεχωρίζω, and ξ is one of the sounds before which the final -ν is normally kept, Δεν ξεχωρίζω is the expected form.

So:

• δε λέω may occur in some contexts/styles
• δεν ξέρω
• δεν μπορώ
• δεν ξεχωρίζω

In your sentence, Δεν is the standard choice.

Yes. You could also say:

• Δεν μπορώ να ξεχωρίσω αυτή τη λέξη από την άλλη όταν μιλάς τόσο γρήγορα.

That means more explicitly:

• I can’t distinguish this word from the other when you speak so fast.

The original sentence with just Δεν ξεχωρίζω is slightly more direct and idiomatic in many contexts. It sounds like a straightforward statement of inability in practice, even without μπορώ.

So both are possible, but:

• Δεν ξεχωρίζω... = very natural, direct
• Δεν μπορώ να ξεχωρίσω... = explicitly stresses inability