أضع الحليب في الثلاجة قبل أن أذهب إلى العمل.

Because the verb itself already tells you the subject.

أضع means I put / I am putting. The prefix أـ marks first person singular in the present tense.

So Arabic often does not need أنا here.
You could say أنا أضع الحليب..., but that would usually add emphasis, like I put the milk... or As for me, I put the milk...

It is the present tense, but in Arabic the present tense often covers several meanings that English separates:

• I put
• I am putting
• I usually put

In this sentence, the most natural meaning is often habitual:

I put the milk in the fridge before I go to work.

So it sounds like something the speaker normally does.

Arabic often uses the definite article with mass nouns or when the item is understood from context.

So الحليب can mean:

• the milk in a specific situation
• or sometimes simply milk as a known substance in context

In English, we often say just milk, but Arabic commonly says الحليب.

So in this sentence, الحليب is perfectly natural.

في means in, and الثلاجة means the refrigerator / the fridge.

So:

• في = in
• الثلاجة = the fridge

Together: في الثلاجة = in the fridge

Also, after a preposition like في, the noun is grammatically in the genitive case. In normal Arabic writing, the short case vowel is usually not written, but in full vocalization it would be:

في الثَّلَّاجَةِ

After قبل when you want to say before I go / before I do..., Arabic commonly uses أن + present verb.

So:

• قبل = before
• أن أذهب = that I go / for me to go, but more naturally just I go

Together:

قبل أن أذهب = before I go

Without أن, the sentence would not sound correct in standard Arabic in this structure.

Yes, it does change grammatically, but the change is often not visible in normal spelling.

After أن, the present-tense verb is usually in the subjunctive.

So:

• indicative: أذهبُ
• subjunctive after أن: أذهبَ

In ordinary Arabic writing, the final short vowel is usually omitted, so both are written the same way:

أذهب

But if fully vocalized, the sentence would show the difference:

قبلَ أن أذهبَ إلى العملِ

Yes, literally it is something like to the work, but idiomatically it means to work.

In Arabic, abstract nouns and common destinations often appear with الـ even when English does not use the.

So:

• إلى العمل = to work
• literally: to the work

This is completely natural Arabic.

Yes, absolutely.

That would mean:

before going to work

So you have two natural options:

• قبل أن أذهب إلى العمل = before I go to work
• قبل الذهاب إلى العمل = before going to work

The first uses a full verb clause.
The second uses a verbal noun / noun-like expression.

Both are correct, but قبل أن أذهب sounds a bit more directly verbal and personal.

Yes, you can.

Both are correct:

• أضع الحليب في الثلاجة...
• أنا أضع الحليب في الثلاجة...

The version without أنا is more neutral and very common, because the verb already shows the subject.

Adding أنا can give:

• emphasis
• contrast
• clarification

For example:

أنا أضع الحليب في الثلاجة، لكن أخي يتركه على الطاولة.
I put the milk in the fridge, but my brother leaves it on the table.

Because ث is a sun letter.

When الـ comes before a sun letter, the l sound of الـ is assimilated into the next consonant.

So:

• written: الثلاجة
• pronounced: ath-thallāja

The ث gets emphasized, and the ل of the article is not pronounced separately.

This is the same pattern you see in words like:

• الشمس pronounced ash-shams
• الناس pronounced an-nās

It would be:

أَضَعُ الحليبَ في الثَّلَّاجَةِ قبلَ أن أذهبَ إلى العملِ.

This shows the grammatical endings more clearly:

• أَضَعُ = I put
• الحليبَ = direct object, accusative
• الثَّلَّاجَةِ = after في, genitive
• قبلَ = often accusative as an adverbial expression
• أذهبَ = subjunctive after أن
• العملِ = after إلى, genitive

In normal everyday writing, most of these short vowels are omitted.

This is a very common learner question.

The dictionary form is وَضَعَ = to put.
But in the present tense, this verb is irregular because it is a weak verb.

Its present forms are built like this:

• أَضَعُ = I put
• تَضَعُ = you put / she puts
• يَضَعُ = he puts
• نَضَعُ = we put

So the initial و of the root does not appear in these present forms.

This is something you simply learn as part of the verb pattern.

Beginning with the verb is very normal in Arabic.

So أضع الحليب... is a natural verbal sentence.

Arabic often likes:

• verb + subject + rest
• or, when the subject is already built into the verb, just verb + rest

If you start with أنا, that is also correct, but it slightly shifts the focus toward the subject.

So:

• أضع الحليب... = neutral, natural
• أنا أضع الحليب... = more emphasis on I