מסיבת ההחלפות מעודדת שימוש חוזר.

A common transliteration is:

mesibat hahakhlafot me'odedet shimush chozer

A few pronunciation notes:

• ח in החלפות and חוזר is a throaty sound, not a regular English h.
• ע in מעודדת is often very weak in modern Israeli Hebrew, so learners may hear me-odedet.
• Hebrew is usually stressed near the end of the word, so listening to native audio is especially helpful here.

Because מסיבת is the construct form of מסיבה.

Hebrew often links two nouns in a structure called סמיכות, roughly like X of Y. In that structure, the first noun may change form.

So:

• מסיבה = a party
• מסיבת = party of ...

Then:

• מסיבת ההחלפות = the swap party / the exchange party

A very common pattern is:

• feminine noun ending in ־ה
• changes to ־ת in construct

For example:

• מתנה → מתנת
• שנה → שנת
• מסיבה → מסיבת

Yes. In a construct phrase, the last noun usually carries the definiteness for the whole expression.

So:

• מסיבת החלפות = a swap party
• מסיבת ההחלפות = the swap party

Even though מסיבת itself does not have ה־, the whole phrase is definite because ההחלפות is definite.

This is very different from English, so it is a common point of confusion.

Because Hebrew naturally uses the plural noun here: swaps / exchanges.

So literally, the phrase is something like:

• party of the exchanges/swaps

But in natural English, we usually say:

• swap party
• exchange party

Hebrew often uses a plural noun in places where English uses a singular noun as a kind of modifier.

Because it agrees with the subject, and the subject is מסיבת ההחלפות, which is feminine singular.

In Hebrew, present-tense verb forms behave a lot like participles, so they agree in:

• gender
• number

Here:

• מסיבה is feminine singular
• therefore מעודדת is feminine singular

Compare:

• האירוע מעודד = the event encourages
• המסיבה מעודדת = the party encourages
• המסיבות מעודדות = the parties encourage

In modern Hebrew, present-tense forms can function in a way that feels partly verbal and partly adjectival.

Here, מעודדת is functioning as the present-tense verb form of לעודד = to encourage.

So in this sentence it means:

• encourages
• or, depending on context, is encouraging

But in this specific sentence, the natural English translation is usually encourages, because it describes a general effect or function of the party.

Yes.

Hebrew present tense often covers both meanings that English separates:

• encourages
• is encouraging

So:

• מסיבת ההחלפות מעודדת שימוש חוזר

can be understood as a general statement: the swap party encourages reuse.

English usually chooses encourages here because it sounds more natural for a general truth or repeated effect.

Because את marks a definite direct object, and שימוש חוזר here is being used in a general, indefinite sense.

So:

• מעודדת שימוש חוזר = encourages reuse

No את is needed.

If the object were definite, you could get:

• מעודדת את השימוש החוזר = encourages the reuse / encourages reuse in a more specific, definite sense

A good basic rule is:

• use את before a definite direct object
• do not use it before an indefinite one

Because חוזר is an adjective here, and Hebrew adjectives usually come after the noun.

So:

• שימוש = use
• חוזר = חוזר = repeated / returning

Together:

• שימוש חוזר = reuse, repeated use

This is a normal noun + adjective pattern in Hebrew.

Also, the adjective agrees with the noun:

• שימוש is masculine singular
• so חוזר is masculine singular

Not exactly.

• שימוש חוזר = reuse
  using the same item again

• מיחזור = recycling
  processing material so it can become something new

Example:

• using the same glass jar again = שימוש חוזר
• turning plastic into new material = מיחזור

They are related ideas, but they are not the same word.

A very literal breakdown is:

• מסיבת = party of
• ההחלפות = the exchanges / the swaps
• מעודדת = encourages
• שימוש חוזר = reuse

So word-for-word, it is roughly:

The swap party encourages reuse.

Or more literally:

The party of the swaps encourages repeated use.

The natural English version is smoother than the literal one, but the Hebrew grammar becomes much clearer when you break it apart this way.