أتمنى طقسا جميلا غدا.

أتمنى means I hope or I wish.

It is the first-person singular form of the verb تمنّى (to wish / to hope for). The initial أ marks I in the present/imperfect tense.

So:

• أتمنى = I hope / I wish

It is pronounced roughly atamannā.

Because in Arabic, the verb تمنّى can take a direct object without a preposition.

So Arabic says:

• أتمنى طقسًا جميلًا
  literally: I hope/wish nice weather

In natural English, we often say I hope for nice weather, but Arabic does not need a separate word matching for here.

Those endings show the accusative case in fully vocalized Arabic.

• طقسًا is the direct object of أتمنى
• جميلًا is the adjective describing طقسًا, so it matches it in case

So both are indefinite accusative:

• طقسًا = weather (as a direct object)
• جميلًا = beautiful / nice (matching طقسًا)

In normal everyday writing, these case endings are often omitted, but they are part of the underlying grammar.

Because in Arabic, adjectives normally come after the noun they describe.

So:

• طقسًا جميلًا = nice weather

This is the normal Arabic order:

• noun + adjective

not:

• adjective + noun

Because the meaning is nice weather, not the nice weather.

In Arabic, if the noun is indefinite, the adjective must also be indefinite.

So:

• طقسٌ جميلٌ / طقسًا جميلًا = nice weather
• الطقس الجميل = the nice weather / the beautiful weather

Arabic adjectives must match the noun in definiteness as well as in gender, number, and case.

Because طقس is a masculine singular noun in Arabic.

The adjective must agree with it, so you get:

• طقس جميل

not:

• طقس جميلة

Even though English nouns like weather do not have grammatical gender, Arabic nouns do, and طقس is masculine.

غدًا means tomorrow.

Here it functions as an adverb of time: it tells you when the hoped-for nice weather is expected.

So the sentence structure is basically:

• أتمنى = I hope
• طقسًا جميلًا = nice weather
• غدًا = tomorrow

Together: I hope for nice weather tomorrow.

Because غدًا is commonly used in the accusative as an adverbial expression of time.

This is very common in Arabic with words that indicate time or place.

So the -an ending here is not because غدًا is the direct object, but because it is a time adverb in the accusative.

Arabic often expresses this idea more compactly than English.

English often says:

• I hope the weather will be nice tomorrow

But Arabic can shorten that to:

• أتمنى طقسًا جميلًا غدًا

This is like saying:

• I hope for nice weather tomorrow

If you want a fuller structure closer to English, you can say:

• أتمنى أن يكون الطقس جميلًا غدًا
  = I hope that the weather will be nice tomorrow

Both are understandable; the longer version is more explicit.

Both are natural, but they feel slightly different.

• أتمنى طقسًا جميلًا غدًا is shorter and more compact
• أتمنى أن يكون الطقس جميلًا غدًا is more explicit and closer to how English packages the idea

A learner will often find the longer version easier to understand grammatically, because it contains the full clause:

• that the weather will be nice tomorrow

But the shorter sentence is perfectly normal and elegant.

That extra ا is a spelling convention used with many words that take tanwīn fatḥ (-an) in the accusative.

So:

• طقسًا is pronounced ṭaqsan
• جميلًا is pronounced jamīlan
• غدًا is pronounced ghadan

The written ا does not mean you pronounce a long ā at the end. It is mainly an orthographic marker for the -an ending.

Yes, Arabic often allows some flexibility in word order.

For example, you could also say:

• أتمنى غدًا طقسًا جميلًا

But أتمنى طقسًا جميلًا غدًا is the most straightforward and neutral order.

Placing غدًا later keeps the sentence easy to process:

• verb
• object
• time expression

That is a very common pattern in Arabic.