אני שמחה שחגגנו ביחד, כי הייתה מסיבה פשוטה אבל מאוד טובה.

In Hebrew, the verb to be is usually left out in the present tense.

So אני שמחה literally looks like I happy, but it means I am happy / glad.

Compare:

• אני שמחה = I am happy
• הייתי שמחה = I was happy
• אהיה שמחה = I will be happy

So the missing am is completely normal.

Because שמחה is the feminine singular form, and it agrees with the speaker.

• A woman says: אני שמחה
• A man says: אני שמח

Hebrew adjectives often change form to match gender and number.

The prefix ש־ means that here.

So:

• שחגגנו ביחד = that we celebrated together

In Hebrew, this ש־ is usually attached directly to the next word, instead of being written as a separate word.

It is a very common connector in everyday Hebrew.

חגגנו is a past tense verb meaning we celebrated.

Breakdown:

• root: ח־ג־ג related to celebrating
• ending ־נו often marks we in the past tense

So:

• חגגתי = I celebrated
• חגגת = you celebrated
• חגגנו = we celebrated

Because the verb already tells you who did the action, Hebrew often does not need to say אנחנו.

ביחד means together.
So does יחד.

In modern Hebrew, both are common:

• ביחד
• יחד

The difference is usually small. ביחד often sounds a bit more natural or conversational in everyday speech, but both are correct.

Because כי means because, while ש־ here means that.

This sentence has two different kinds of subordinate clauses:

• אני שמחה שחגגנו ביחד = I’m glad that we celebrated together
• כי הייתה מסיבה... = because there was a party...

So:

• ש־ introduces what someone is happy/glad about
• כי introduces the reason

Because מסיבה is a feminine singular noun, and הייתה is the feminine singular past form of to be.

• היה = masculine singular
• הייתה = feminine singular

Since the sentence says מסיבה (party), which is feminine, Hebrew uses:

• הייתה מסיבה = there was a party

Even though הייתה comes before מסיבה, it still agrees with it.

Hebrew often uses a form of היה / הייתה to express there was / there were.

So:

• הייתה מסיבה literally = was a party
• natural English = there was a party

Hebrew does not need a separate word exactly like English there in this structure.

Hebrew has no indefinite article. That means there is no separate word for a or an.

So:

• מסיבה can mean a party
• המסיבה means the party

This is very common in Hebrew:

• ספר = a book
• הספר = the book

Because in Hebrew, adjectives usually come after the noun.

So:

• מסיבה פשוטה = a simple party
• מסיבה טובה = a good party

This is the normal Hebrew word order.

Also, the adjectives must agree with מסיבה, which is feminine singular, so we get:

• פשוטה and not פשוט
• טובה and not טוב

Yes, טובה מאוד is also possible.

מאוד means very, and in modern Hebrew it can appear before or after the adjective:

• מאוד טובה
• טובה מאוד

Both are natural. In this sentence, מאוד טובה fits smoothly inside the phrase מסיבה פשוטה אבל מאוד טובה.

So the important thing to notice is:

• the noun still comes first: מסיבה
• the adjective phrase comes after it: מאוד טובה