أن أمشي في الحديقة أفضل من أن أجلس في البيت عندما يكون الطقس جميلا.

Here, أن is not best understood as that. It is the particle that introduces a verb in the subjunctive and often corresponds to English to or for ... to.

So:

• أن أمشي = to walk / for me to walk
• أن أجلس = to sit / for me to sit

This is different from أنّ meaning that, which is normally followed by a noun or a nominal clause, not directly by a finite verb.

After أن, the present tense verb goes into the subjunctive.

With full vowel marks, you would have:

• أن أمشيَ
• أن أجلسَ

In ordinary Arabic writing, short vowels are usually not written, so learners often do not see the change clearly.

A rough comparison:

• أجلسُ = I sit / I am sitting (indicative)
• أن أجلسَ = to sit / that I sit (subjunctive after أن)

The sentence is basically a nominal sentence whose subject is an entire verb phrase introduced by أن.

You can think of it like this:

• أن أمشي في الحديقة = the thing being talked about, literally for me to walk in the garden
• أفضل من أن أجلس في البيت = the comment about it, is better than sitting in the house

So the main pattern is:

• [أن + verb phrase] + أفضل من + [أن + verb phrase]

This is a common Arabic way to compare actions.

Because Arabic is comparing two actions, and each action is expressed with its own أن + verb phrase:

• أن أمشي في الحديقة
• أن أجلس في البيت

This makes the comparison nicely parallel:

• to walk ... is better than to sit ...

If you removed the second أن, the structure would not work properly.

Arabic can also express the same idea with verbal nouns:

• المشي في الحديقة أفضل من الجلوس في البيت

That version is also very natural.

Yes, من often means from, but after a comparative word like أفضل it means than.

So:

• أفضل من = better than
• أكبر من = bigger than
• أسرع من = faster than

In this sentence:

• أفضل من أن أجلس في البيت = better than sitting in the house

أفضل is the comparative form meaning better / best.

In comparisons like this, Arabic commonly uses the masculine singular form as the default, especially when what is being compared is not a simple noun but a whole action or clause.

So even though the subject is an entire phrase (أن أمشي في الحديقة), Arabic still uses:

• أفضل

not some special agreeing form.

Because in Arabic, the present-tense copula is/am/are is usually not stated in nominal sentences.

So:

• أن أمشي في الحديقة أفضل...

already means:

• Walking in the garden is better...

Arabic often leaves present-tense is unstated.
That is completely normal.

Literally, it is personal, because the verbs are first person singular:

• أمشي = I walk
• أجلس = I sit

So the sentence most directly means something like:

• For me, walking in the garden is better than sitting at home when the weather is nice.

If you wanted a more general statement, Arabic would often use verbal nouns instead:

• المشي في الحديقة أفضل من الجلوس في البيت عندما يكون الطقس جميلاً

That sounds more like Walking in the garden is better than sitting at home... in a general sense.

Because في means in or inside, while إلى means to.

So:

• أمشي في الحديقة = I walk in the garden / around the garden
• أمشي إلى الحديقة = I walk to the garden

Those are different ideas.

Also, في البيت often naturally means at home, even though the literal wording is in the house.

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

Here:

• في الحديقة can mean in the garden
• في البيت can mean in the house / at home
• الطقس = the weather

With words like weather, Arabic normally uses the definite form.
And with places like البيت, the definite form is very common in everyday expression.

So this is not unusual at all.

عندما usually introduces a time clause, and Arabic very often follows it with a verbal clause.

Since الطقس جميل is a nominal sentence (the weather is nice), Arabic commonly inserts يكون to make the time clause sound natural:

• عندما يكون الطقس جميلاً = when the weather is nice

So يكون here works like is / becomes / is being, depending on context.

This whole phrase gives the condition or time setting for the comparison: walking in the garden is better than sitting at home when the weather is nice.

There are two things going on:

• جميل is the correct adjective because الطقس is masculine singular.
• It appears as جميلاً with fatḥa/tanwīn fatḥ because it is the predicate of يكون, and the predicate of كان / يكون is in the accusative.

So the grammar is:

• يكون = the verb
• الطقسُ = the subject of يكون (nominative)
• جميلاً = the predicate of يكون (accusative)

That is why the fully vocalized form is:

• عندما يكون الطقسُ جميلاً