في المساء جلست في الغرفة لأنها كانت هادئة، وكتبت لصديقتي عن الدعوة الجميلة.

Arabic often places a time expression first to set the scene. في المساء literally means in the evening, and it works a lot like an opening time phrase in English, such as In the evening, ...

So:

في المساء جلست في الغرفة...
= In the evening, I sat in the room...

This is very natural in Arabic.

Because the verb itself already contains the subject.

In the past tense, Arabic verbs show who did the action. So جلستُ means I sat, and كتبتُ means I wrote. Since I is already built into the verb, Arabic usually does not need أنا.

So:

• جلستُ = I sat
• كتبتُ = I wrote

You could add أنا for emphasis, but normally it is omitted.

This is a very common learner question, because normal Arabic writing usually leaves out short vowels.

Without vowel marks, جلست could represent several forms:

• جلستُ = I sat
• جلستَ = you (masculine singular) sat
• جلستِ = you (feminine singular) sat
• جلستْ = she sat

The same is true for كتبت.

So how do you know? From context. If the meaning has already been given to you as I sat ... and I wrote ..., then you understand these as جلستُ and كتبتُ.

لأنها breaks down into two parts:

• لأن = because
• ها = she / it (feminine singular pronoun)

So لأنها literally means because she/it...

In this sentence, ها refers back to الغرفة (the room), which is a grammatically feminine noun in Arabic. So Arabic uses the feminine pronoun form.

That is why:

لأنها كانت هادئة
literally = because it/she was quiet
natural English = because it was quiet

Because Arabic grammar and natural gender are not always the same thing.

The noun غرفة (room) is grammatically feminine, so words that refer back to it also use feminine forms:

• الغرفة = the room
• لأنها = because it/she...

This does not mean Arabic speakers think a room is biologically female. It just means the noun belongs to the feminine grammatical class.

Because هادئة is the feminine form of the adjective, and it describes الغرفة, which is feminine.

• هادئ = quiet (masculine)
• هادئة = quiet (feminine)

So:

• ولد هادئ = a quiet boy
• غرفة هادئة = a quiet room

Arabic adjectives must agree with the noun they describe in gender.

كانت is the past-tense form of كان, and here it means was.

Arabic often leaves out the verb to be in the present tense:

• الغرفة هادئة = the room is quiet

But in the past, Arabic normally uses كان:

• الغرفة كانت هادئة = the room was quiet

So كانت هادئة means was quiet.

Because Arabic prepositions are often attached directly to the following word.

Here:

• لِـ = to / for
• صديقتي = my female friend

Together they are written as one word:

لصديقتي = to my female friend

This is completely normal in Arabic. Many short prepositions attach this way.

Also, with the verb كتب, Arabic commonly uses لِـ for write to someone:

• كتبتُ لصديقتي = I wrote to my friend

صديقتي means my female friend.

It breaks down like this:

• صديقة = female friend
• ـي = my

So:

صديقتي = my female friend

Compare:

• صديقي = my male friend
• صديقتي = my female friend

The sentence tells you the friend is female, but it does not tell you whether the speaker is male or female.

Here عن means about.

So:

كتبت لصديقتي عن الدعوة الجميلة
= I wrote to my friend about the beautiful invitation

This is a very common use of عن:

• تحدثت عن... = I spoke about...
• قرأت عن... = I read about...
• كتبت عن... = I wrote about...

Because in Arabic, adjectives normally come after the noun.

So:

• الدعوة الجميلة = the beautiful invitation

not

• الجميلة الدعوة

Arabic adjectives also agree with the noun in several ways. Here الجميلة matches الدعوة in:

• gender: both feminine
• number: both singular
• definiteness: both have الـ

That is why both words are definite:

• دعوة جميلة = a beautiful invitation
• الدعوة الجميلة = the beautiful invitation

Because Arabic adjectives agree with the noun in definiteness as well as gender and number.

So if the noun is definite, the adjective is also definite:

• الدعوة الجميلة = the beautiful invitation

If the noun is indefinite, the adjective is indefinite too:

• دعوة جميلة = a beautiful invitation

This matching is one of the most important adjective rules in Arabic.

Yes. In fully vocalized, fully inflected MSA, they would.

A more fully marked version would be:

في المساءِ جلستُ في الغرفةِ لأنها كانت هادئةً، وكتبتُ لصديقتي عن الدعوةِ الجميلةِ.

For example:

• after في, nouns are normally genitive:
  المساءِ, الغرفةِ
• after عن, nouns are also genitive:
  الدعوةِ الجميلةِ
• the predicate after كانت is normally accusative:
  هادئةً

But in ordinary Arabic writing, these short vowels are usually not written, so learners must get used to reading forms without them.

This letter is called taa marbuuTa.

Usually, when you stop on the word, it sounds like a final -a:

• الغرفة ≈ al-ghurfa
• الدعوة ≈ ad-da‘wa
• هادئة ≈ haadi'a
• الجميلة ≈ al-jamiila

But when the word is grammatically connected in full pronunciation, the t sound can appear:

• في الغرفةِ = fii al-ghurfati
• الدعوةُ الجميلةُ = ad-da‘watu al-jamiilatu

So learners often hear it as -a in pause, but grammatically it is still a t-type ending.