Δεν ξέρω κανέναν λογιστή που να απαντάει τόσο γρήγορα σε email.

Because λογιστή is the direct object of ξέρω.

• ξέρω = I know
• The person you know is the object, so Greek uses the accusative
• λογιστής is nominative: ο λογιστής
• λογιστή is accusative: τον λογιστή

The same applies to κανέναν:

• κανείς = nominative
• κανέναν = accusative masculine singular

So:

• Δεν ξέρω κανέναν λογιστή = I don’t know any accountant

In a negative sentence, κανέναν means anyone / anybody / no one, depending on how you translate it naturally into English.

So:

• Δεν ξέρω κανέναν λογιστή
  literally: I don’t know no accountant natural English: I don’t know any accountant / I know no accountant

This is a very important Greek pattern: negative words normally appear together with δεν.

So Greek often uses what looks like a double negative to an English speaker, but in Greek that is completely normal:

• Δεν είδα κανέναν = I didn’t see anyone
• Δεν έχω τίποτα = I don’t have anything

Because the speaker is talking about a non-specific, hypothetical person with a certain characteristic.

κανέναν λογιστή does not refer to one known accountant. It means any accountant at all. After this kind of negative or indefinite idea, Greek very often uses:

που να + verb

This has the sense of:

• who would
• who might
• who could
• who is the kind of person to

So:

• κανέναν λογιστή που να απαντάει τόσο γρήγορα = any accountant who answers / would answer that quickly

Using που να shows that such a person is being described as a type, not as a specific known individual.

Yes, this is a να-construction, which is how Modern Greek expresses what is traditionally called the subjunctive.

In Modern Greek, the particle να is what signals that kind of meaning. So:

• απαντάει by itself can be a present-tense form
• να απαντάει = a subjunctive-type construction

Here it is also imperfective, which fits the idea of a general or repeated behavior:

• an accountant who answers quickly in general
• not just one single reply on one occasion

So the sentence is about a habitual quality, not one isolated event.

Both are correct.

For many verbs in Modern Greek, especially in everyday language, you may see two forms like this:

• απαντά
• απαντάει

They mean the same thing here. The longer form απαντάει is very common in speech and informal writing.

So:

• που να απαντά τόσο γρήγορα
• που να απαντάει τόσο γρήγορα

Both are acceptable.

Both can work, but they are not exactly the same.

• πολύ γρήγορα = very quickly
• τόσο γρήγορα = so quickly / that quickly

τόσο emphasizes the degree more strongly. It often has the feeling of:

• as quickly as that
• so unusually quickly

So in this sentence, τόσο γρήγορα sounds a bit more expressive and emphatic.

Because email here is being used in a general sense.

The phrase means something like:

• in email
• by email
• to emails
• when it comes to emails

Greek often omits the article in this kind of general, somewhat fixed expression, especially with loanwords and informal modern vocabulary.

Also, email is commonly treated as an indeclinable loanword, so its form often stays the same.

Depending on context, Greek speakers might also say things like:

• στα email = in the emails / to emails
• σε ένα email = in an email / to an email

But σε email is a natural general expression.

This is a very normal Greek word order:

• Δεν ξέρω = verb first
• κανέναν λογιστή = object
• που να απαντάει... = relative clause describing λογιστή

So the structure is:

I don’t know + any accountant + who...

Greek word order is flexible, but this order is clear and natural because the relative clause comes right after the noun it describes:

• λογιστή → που να απαντάει τόσο γρήγορα σε email

That makes it easy to understand that που να απαντάει... describes the accountant.

Yes. γνωρίζω is possible, but it sounds a bit more formal or careful.

Compare:

• Δεν ξέρω κανέναν λογιστή... = very natural, everyday Greek
• Δεν γνωρίζω κανέναν λογιστή... = more formal

In ordinary conversation, ξέρω is extremely common for this kind of meaning.

Because nouns ending in -τής usually form the accusative singular in -τή.

So:

• ο λογιστής = the accountant
• τον λογιστή = the accountant (object form)

Other similar examples:

• ο φοιτητής → τον φοιτητή
• ο καθηγητής → τον καθηγητή

So κανέναν λογιστή follows a very common masculine noun pattern.