Ο αρραβώνας τους θα γίνει τον Μάιο στο σπίτι των γονιών της.

Greek uses the definite article much more often than English. So ο αρραβώνας literally means the engagement, and in a sentence like this the article sounds natural.

Also, αρραβώνας is a masculine singular noun, so its nominative singular article is ο.

αρραβώνας is in the nominative singular because it is the subject of the sentence:

• Ο αρραβώνας τους = their engagement

In Greek, the subject of the sentence normally appears in the nominative.

Here τους means their.

Greek often uses short possessive pronouns after the noun they belong to:

• ο αρραβώνας τους = their engagement
• literally: the engagement their

That is a normal Greek pattern. English puts the possessive before the noun, but Greek often puts it after.

Because they refer to different owners:

• τους = their → their engagement
• της = her → her parents

So the sentence means that the engagement belongs to them, but the house belongs to her parents.

θα γίνει is the future form of γίνομαι.

In this sentence, it means something like:

• will happen
• will take place
• will be held

So although γίνομαι can often mean become, here it clearly means take place / happen because the subject is the engagement.

Because after θα, Greek normally uses the subjunctive-type form rather than the ordinary present form.

• γίνεται = it happens / is happening
• θα γίνει = it will happen / it will take place

So θα γίνει is the correct future expression here.

Greek often uses the accusative for expressions of time, especially with months, dates, and duration.

So:

• ο Μάιος = May (nominative)
• τον Μάιο = in May (accusative)

This is a very common Greek structure. Even though English uses in, Greek can simply use the accusative here.

Because Greek usually includes the article with months in expressions like this.

So Greek says:

• τον Μάιο = in May

That article is normal and expected. English does not use the here, but Greek does.

στο is a contraction of:

• σε + το = στο

So:

• στο σπίτι = at the house / in the house

The preposition σε often means in, at, to, depending on context.

Also, σπίτι is a neuter noun. Its nominative and accusative forms are the same, so after στο it still looks like σπίτι.

This is a genitive phrase showing possession:

• των γονιών = of the parents
• της = her

So:

• στο σπίτι των γονιών της = at her parents' house
• literally: at the house of her parents

The word των is the genitive plural article, used because γονιών is plural and possessive here.

Because γονείς is the nominative plural form, while here Greek needs a genitive plural form after των.

So:

• οι γονείς = the parents
• των γονιών = of the parents

This is the correct case change for the structure used in the sentence.

In other contexts, της can have different grammatical functions, but in this sentence it is clearly a possessive pronoun meaning her.

So:

• των γονιών της = her parents'

The context makes that meaning clear.

Greek word order is fairly flexible. This sentence has a neutral, natural order, but other orders are possible for emphasis.

For example:

• Τον Μάιο θα γίνει ο αρραβώνας τους στο σπίτι των γονιών της.
• Στο σπίτι των γονιών της θα γίνει τον Μάιο ο αρραβώνας τους.

These variations can sound slightly different in emphasis, but the basic meaning stays the same because the cases and articles show the relationships clearly.

Both are possible, depending on how natural you want the English to sound.

For an event like αρραβώνας, Greek often uses γίνομαι, where English might say:

• will take place
• will be held
• will happen

So in this sentence, will take place or will be held is often the most natural English choice.

Yes. English has several natural ways to express it:

• at her parents' house
• in her parents' house
• at her parents' home
• in her parents' home

The Greek phrase itself is broad enough to allow these, and the best English version depends on style and context.