الحديقة جميلة خلال النهار.

Because it describes الحديقة (the garden), and حديقة is a feminine noun in Arabic. Adjectives in Arabic must agree with the nouns they describe in gender, number, and definiteness.

So:

• الحديقة = feminine, singular, definite
• جميلة = feminine, singular, and it is understood as matching a definite noun

That is why you use جميلة and not جميل.

This sentence is a nominal sentence in Arabic, meaning it begins with a noun rather than a verb.

• الحديقة = the subject/topic
• جميلة = the predicate/description

So the structure is:

• الحديقة = The garden
• جميلة = is beautiful

Arabic often says things like The garden beautiful without using a separate word for is in the present tense.

It is not written in Arabic in the present tense.

So:

• الحديقة جميلة literally = The garden beautiful
• natural English = The garden is beautiful

In Modern Standard Arabic, present-tense to be is usually omitted in simple sentences like this.

Because النهار means the daytime or the day here, and it is being used as a definite time expression.

• نهار = daytime / day
• النهار = the daytime

In the phrase خلال النهار, the meaning is during the day or during daytime.

Arabic often uses the definite article in time expressions where English may or may not use the.

خلال means during or throughout.

So:

• خلال النهار = during the day
• خلال الليل = during the night
• خلال الأسبوع = during the week

It introduces a time period within which something happens or is true.

Yes, often you can, but the nuance is slightly different.

• في النهار = in the daytime / during the day
• خلال النهار = during the day / throughout the day

In many everyday contexts, both are acceptable.
However, خلال can sound a little more formal or a little more focused on the span of time.

So:

• الحديقة جميلة في النهار = The garden is beautiful in the daytime.
• الحديقة جميلة خلال النهار = The garden is beautiful during the day.

Both are good, but خلال is the exact word in this sentence.

The final ة is called tāʾ marbūṭa. In pause, it is usually pronounced like -a or -ah, not -t.

So:

• الحديقة is pronounced roughly al-ḥadīqa
• not al-ḥadīqat when stopping there normally

But the t sound can appear in certain grammatical contexts, especially when endings are fully pronounced in careful reading or when the word is in an iḍāfa construction.

It is pronounced roughly jamīlah.

Breakdown:

• ج = j in standard MSA pronunciation
• مي = mī
• لة = lah when stopping

So the whole word is:

• جميلة = jamīlah = beautiful

Because it ends in ة, the final sound in pause is usually -ah, not -t.

In fully vowelled formal grammar, yes: when an adjective modifies a definite noun directly, it is also definite.

So the fully matched adjective would be:

• الحديقة الجميلة = the beautiful garden

But in your sentence, جميلة is not being used as an adjective inside a noun phrase. It is the predicate of the sentence:

• الحديقة جميلة = The garden is beautiful

In predicate position, it normally does not take الـ.

So compare:

• الحديقة الجميلة = the beautiful garden
• الحديقة جميلة = the garden is beautiful

Yes, in fully vocalized formal Arabic there are case endings, though they are often not written in normal text and often not pronounced in everyday speech.

The full form would be approximately:

• الحديقةُ جميلةٌ خلالَ النهارِ

Explanation:

• الحديقةُ: subject of the nominal sentence, so nominative
• جميلةٌ: predicate, also nominative
• خلالَ: often takes accusative as an adverb-like ظرف
• النهارِ: after خلال, it is in the genitive

In ordinary writing, you usually see:

• الحديقة جميلة خلال النهار

without the short vowels.

A helpful breakdown is:

• الحديقة = the garden
• جميلة = beautiful
• خلال = during
• النهار = the day / daytime

So literally:

• The garden beautiful during the daytime

Natural English:

• The garden is beautiful during the day.

This is a very normal Arabic structure: noun + description + time phrase.