صديقتي تستطيع أن تمشي إلى الحديقة إذا كان الطقس جميلا.

It means my female friend.

• صديقة = female friend
• ـي = my

So صديقتي literally means my female friend. In some contexts it can also mean my girlfriend, but the basic meaning is my female friend.

Also, when a word ending in ة takes a suffix, that ending is pronounced as t, so صديقة + ي becomes صديقتي.

Because the subject, صديقتي, is feminine singular.

• هو يستطيع = he can / he is able
• هي تستطيع = she can / she is able

Since صديقتي refers to a woman or girl, the verb has to match it, so تستطيع is the correct form.

After تستطيع in Modern Standard Arabic, it is very common to use أن + present verb.

So:

• تستطيع أن تمشي = she can walk
• literally, something like she is able to walk

This is a very standard pattern in MSA:

• أستطيع أن أقرأ = I can read
• نستطيع أن نذهب = we can go

Because normal Arabic writing usually leaves out short vowels.

After أن, the verb is in the subjunctive, so in full vowelled Arabic it would be:

• أن تمشيَ

But without short vowels, it is still written simply as:

• أن تمشي

So the grammar changes, but the spelling often does not visibly change.

Arabic allows both noun-first and verb-first sentences.

This sentence begins with صديقتي to make my friend the topic:

• صديقتي تستطيع... = My friend can...

A verb-first version is also possible:

• تستطيع صديقتي أن تمشي...

Both are understandable. The noun-first version can feel a bit more like emphasizing my friend as the topic of the sentence.

إلى means to in the sense of movement toward a place.

So:

• تمشي إلى الحديقة = she walks to the park/garden

It is the normal preposition to use with movement toward a destination.

حديقة usually means garden, but in many contexts it can also be translated as park.

The ال makes it definite:

• حديقة = a garden / a park
• الحديقة = the garden / the park

So here it means the park or the garden, depending on context.

إذا is commonly used for a real or likely condition, especially one connected to the present or future.

So:

• إذا كان الطقس جميلا = if the weather is nice

This is different from لو, which is often used for unreal or contrary-to-fact situations.

A rough comparison:

• إذا = if / when for something possible or expected
• لو = if for something unreal or hypothetical

This is a very common Arabic structure.

The basic nominal sentence is:

• الطقس جميل = the weather is nice

After إذا, Arabic very often uses:

• إذا كان الطقس جميلا

Even though كان often means was in other contexts, here the whole phrase is understood as a present or future condition:

• if the weather is nice

So in this sentence, كان does not force a past meaning in English.

There are two reasons.

• الطقس is masculine, so the adjective must be جميل, not جميلة
• after كان, the predicate becomes accusative, so جميل becomes جميلاً

So:

• جميلة would be wrong because it is feminine
• جميل is the basic masculine form
• جميلاً is the correct form here after كان

In normal unvowelled writing, this often appears as جميلا.

A vowelled version would look like this:

صديقتي تستطيعُ أن تمشيَ إلى الحديقةِ إذا كانَ الطقسُ جميلاً.

Useful things to notice:

• تستطيعُ: indicative ending
• تمشيَ: subjunctive after أن
• الحديقةِ: genitive because it comes after إلى
• كانَ: the form used in this construction
• الطقسُ: nominative as the subject of كان
• جميلاً: accusative as the predicate of كان

It is pronounced aṭ-ṭaqs, not al-taqs.

That is because ط is a sun letter. With sun letters, the l sound of ال is not pronounced, and the next consonant is doubled in pronunciation.

So:

• الطقس → aṭ-ṭaqs

This is a very common pronunciation rule in Arabic.