Ο καταναλωτής διαβάζει τον τιμοκατάλογο του προμηθευτή.

These are all forms of the definite article, meaning the in English, but Greek changes the article depending on the noun’s gender, number, and case.

In this sentence:

• ο καταναλωτής = the consumer
• τον τιμοκατάλογο = the price list / catalogue
• του προμηθευτή = of the supplier

So English uses just the, but Greek uses different forms:

• ο for masculine singular nominative
• τον for masculine singular accusative
• του for masculine singular genitive

Because they have different grammatical jobs in the sentence.

• ο καταναλωτής is the subject: the person doing the action.
• τον τιμοκατάλογο is the direct object: the thing being read.

Greek marks this difference with case:

• Nominative case for the subject: ο καταναλωτής
• Accusative case for the direct object: τον τιμοκατάλογο

English mostly shows this through word order, but Greek often shows it through endings and article changes.

Because here the supplier is not the direct object. It means of the supplier, so Greek uses the genitive case.

• του προμηθευτή = of the supplier
• τον προμηθευτή would mean the supplier as a direct object

So the structure is:

• τον τιμοκατάλογο = the price list
• του προμηθευτή = of the supplier

Together: the supplier’s price list or the price list of the supplier

διαβάζει is the 3rd person singular present tense of διαβάζω.

That means it can mean:

• he reads
• she reads
• it reads

Depending on context, it can also translate naturally as:

• is reading

So in this sentence, since the subject is ο καταναλωτής, it means the consumer reads / is reading.

Greek often uses the simple present form to cover both:

• reads
• is reading

So διαβάζει can mean either one, depending on context.

English makes a stronger distinction between:

• He reads = habitual
• He is reading = happening now

Modern Greek often does not need to make that distinction unless the context requires it.

You usually learn a Greek noun together with its article, because the article helps show gender.

In this sentence:

• ο καταναλωτής is masculine
• ο τιμοκατάλογος is masculine
• ο προμηθευτής is masculine

A good habit is to memorize nouns like this:

• ο καταναλωτής
• ο τιμοκατάλογος
• ο προμηθευτής

not just the bare noun by itself.

That way, you also get a clue about how the noun will decline in different cases.

Greek nouns change form depending on case.

For example:

• ο τιμοκατάλογος = nominative singular
• τον τιμοκατάλογο = accusative singular

And:

• ο προμηθευτής = nominative singular
• του προμηθευτή = genitive singular

This is very normal in Greek. English has mostly lost this kind of noun inflection, so it often feels new to English speakers.

Greek word order is more flexible than English because case endings help show who is doing what.

The neutral, straightforward order here is:

• Ο καταναλωτής διαβάζει τον τιμοκατάλογο του προμηθευτή.

But Greek could move things around for emphasis, for example:

• Τον τιμοκατάλογο του προμηθευτή διαβάζει ο καταναλωτής.

This still means essentially the same thing, though the emphasis changes.

English is much less flexible because word order carries more of the grammar.

Yes, του can have more than one use.

It can be:

1. the genitive article for a masculine or neuter singular noun

   • του προμηθευτή = of the supplier

2. a possessive pronoun/clitic in other contexts

   • for example, το βιβλίο του = his book

In your sentence, του is functioning as the article in the genitive phrase του προμηθευτή.

A rough pronunciation guide is:

• Ο καταναλωτής → o ka-ta-na-lo-TIS
• διαβάζει → thya-VA-zee
• τον τιμοκατάλογο → ton ti-mo-ka-TA-lo-go
• του προμηθευτή → too pro-mi-thef-TI

A few useful notes:

• δ in Modern Greek sounds like th in this
• θ sounds like th in think
• stress matters a lot in Greek, and the written accent mark shows it:
  • καταναλωτής
  • διαβάζει
  • τιμοκατάλογο
  • προμηθευτή

The accent mark shows which syllable is stressed.

For example:

• καταναλωτής
• διαβάζει
• τιμοκατάλογο
• προμηθευτή

This is important because stress is a real part of the word in Greek. It helps with both pronunciation and sometimes distinguishing forms.

English spelling usually does not show stress clearly, but Greek spelling usually does.

The dictionary forms are usually the nominative singular for nouns and the 1st person singular present for verbs.

So here they are:

• ο καταναλωτής = the consumer
• διαβάζω = to read
• ο τιμοκατάλογος = price list, catalogue
• ο προμηθευτής = supplier

In the sentence, some of these appear in changed forms because of case or conjugation:

• διαβάζω → διαβάζει
• ο τιμοκατάλογος → τον τιμοκατάλογο
• ο προμηθευτής → του προμηθευτή

In a normal sentence like this, yes. Greek uses definite articles more often than English does, and with ordinary nouns they are usually expected.

So:

• Ο καταναλωτής διαβάζει τον τιμοκατάλογο του προμηθευτή.

sounds natural as a complete sentence.

If you remove the articles, it would usually sound less natural or would need a special context, such as headings, notes, labels, or very compressed style.