Η διάρκεια συμβολαίου τελειώνει τον επόμενο μήνα.

Η διάρκεια means the duration or the term/length.

• η = the for a feminine singular noun in the nominative
• διάρκεια = duration, term, length

Here, Η διάρκεια is the subject of the sentence, so it appears in the nominative case.

συμβολαίου is the genitive singular form of συμβόλαιο (contract).

Greek often uses the genitive where English uses of. So:

• διάρκεια συμβολαίου = duration of a contract / contract duration
• διάρκεια του συμβολαίου = duration of the contract

So the ending -ου here shows a relationship like of a/the contract.

That is a very natural question.

Greek can sometimes leave out the article in this kind of formal or administrative-style phrase:

• διάρκεια συμβολαίου = contract duration / duration of contract
• διάρκεια του συμβολαίου = the duration of the contract

If you are talking about one specific contract in ordinary conversation, του συμβολαίου often sounds more explicit and natural. The version without the article is still understandable, but it can sound more like formal written language or a label.

τελειώνει is the 3rd person singular present tense of τελειώνω, meaning to end, finish.

So literally it means:

• it ends
• it is ending

Since the subject is Η διάρκεια (a singular thing), the verb is also singular: τελειώνει.

In Greek, just like in English sometimes, the present tense can be used for a future event when the time is clear from context.

Here, the time expression τον επόμενο μήνα (next month) makes the future meaning obvious.

So Greek can say:

• τελειώνει τον επόμενο μήνα = ends next month

You could also use a future form such as θα τελειώσει, but the present is very common for scheduled or expected future events.

Because μήνας is a masculine noun, and here it appears in the accusative singular.

So the phrase breaks down like this:

• τον = masculine accusative singular the
• επόμενο = masculine accusative singular next
• μήνα = accusative singular of μήνας (month)

All three words agree with each other in gender, number, and case.

Greek often uses the accusative for expressions of time, especially when saying when something happens.

So:

• τον επόμενο μήνα = next month
• την επόμενη εβδομάδα = next week
• την Κυριακή = on Sunday

This is a normal Greek pattern. English usually does not show this with case endings, so it can feel unfamiliar at first.

τελειώνει is understandable and natural enough in general Greek, but for contracts Greeks very often use λήγει (expires, comes to an end), which sounds more formal and more typical for this context.

So you may also hear:

• Το συμβόλαιο λήγει τον επόμενο μήνα.
• Η διάρκεια του συμβολαίου λήγει τον επόμενο μήνα.

So τελειώνει = more general ends/finishes and λήγει = more specific/formal for contracts, deadlines, subscriptions, etc.

Yes. Greek word order is fairly flexible, because the endings and articles help show the grammatical relationships.

For example, you could also say:

• Τον επόμενο μήνα τελειώνει η διάρκεια συμβολαίου.
• Η διάρκεια του συμβολαίου τελειώνει τον επόμενο μήνα.

The most neutral version is usually the one that starts with the subject, but moving τον επόμενο μήνα to the front gives more emphasis to the time.

Many Greek speakers would probably say one of these:

• Το συμβόλαιο λήγει τον επόμενο μήνα.
• Η διάρκεια του συμβολαίου τελειώνει τον επόμενο μήνα.

The first one is shorter and very natural in everyday use. The original sentence is understandable, but it sounds a bit more formal or compressed, especially because of συμβολαίου without του.