عندما أعود إلى البيت، أضع المفتاح فوق الطاولة قرب الباب.

عندما means when and introduces a time clause.

So the sentence is divided like this:

• عندما أعود إلى البيت = when I return home
• أضع المفتاح فوق الطاولة قرب الباب = I put the key on/above the table near the door

It works very much like English when at the start of a sentence.

In Arabic, the imperfect form (often called the present tense) is used for more than just actions happening right now. It can also express:

• habitual actions
• general truths
• future actions, especially after time words like عندما

Here, the meaning is something like a routine or repeated action:

• When I come home, I put the key...

So using أعود and أضع is completely natural.

If you wanted a completed past event, you would normally use past forms instead:

• عندما عدتُ إلى البيت، وضعتُ المفتاح...
• When I returned home, I put the key...

Arabic often leaves out the subject pronoun when the verb already shows who is doing the action.

Both verbs here already tell you the subject is I:

• أعود = I return
• أضع = I put

The prefix أ- marks first person singular in the imperfect.

So أنا is not necessary. You could add it for emphasis, but the normal neutral sentence does not need it.

Both are possible, but they mean slightly different things.

• إلى البيت = home / to the house
• إلى بيتي = to my house / to my home

In Arabic, البيت often means home when the context already makes it obvious whose home is meant. So أعود إلى البيت is a very natural way to say I return home.

Using بيتي would sound more explicit and a bit more personal:

• أعود إلى بيتي = I return to my home

Because إلى is a preposition, and nouns after prepositions take the genitive case in Modern Standard Arabic.

So:

• إلى = preposition
• البيتِ = genitive after إلى

That final -i sound is the genitive ending.

المفتاح means the key, not a key.

Arabic uses the definite article الـ when the speaker has a specific thing in mind. In this sentence, it is the key the speaker normally uses and puts in its usual place.

Compare:

• أضع المفتاح = I put the key
• أضع مفتاحًا = I put a key

The second one sounds less natural here unless you really mean any key, not a particular one.

Because it is the direct object of the verb أضع.

• أضع = I put
• What do I put? المفتاح

So in full vocalization:

• أضعُ المفتاحَ

The final -a sound marks the accusative case.

This is a very common question.

• على usually means on, with clearer contact with the surface
• فوق means above / over / on top of

In everyday meaning, both can sometimes work in sentences like this:

• أضع المفتاح على الطاولة
• أضع المفتاح فوق الطاولة

But the nuance is slightly different:

• على الطاولة strongly suggests the key is resting on the table
• فوق الطاولة can mean on top of the table or more generally above the table

In many contexts, both are acceptable, but على is often the more basic word for on.

قرب means near or nearby, and here it functions as a locative expression.

It is followed by الباب in the genitive:

• قربَ البابِ = near the door

Like فوق, قرب is often treated as a noun used adverbially or as a locative expression. That is why the noun after it comes in the genitive.

So:

• فوقَ الطاولةِ
• قربَ البابِ

follow a similar pattern.

In context, the most natural understanding is:

• the key is put on the table that is near the door

So semantically, English speakers often hear it as modifying الطاولة:

• on the table near the door

But Arabic often strings location expressions together, so the phrase can also be understood more broadly as part of the whole location of the action.

If you wanted to make the table near the door more explicit, you could say:

• فوق الطاولة القريبة من الباب

That removes the ambiguity.

A fully vocalized version would be:

عِنْدَما أَعُودُ إِلَى البَيْتِ، أَضَعُ المِفْتَاحَ فَوْقَ الطَّاوِلَةِ قُرْبَ البَابِ.

The endings are there for grammatical reasons:

• أعودُ: indicative imperfect
• البيتِ: genitive after إلى
• أضعُ: indicative imperfect
• المفتاحَ: direct object, so accusative
• فوقَ: locative expression, often analyzed as accusative
• الطاولةِ: genitive after فوق
• قربَ: locative expression, often analyzed as accusative
• البابِ: genitive after قرب

In normal everyday writing, these short vowels are usually omitted.

Because ط is a sun letter, while ب is a moon letter.

That means:

• الطاولة is pronounced aṭ-ṭāwila, with the l sound of الـ assimilated
• الباب is pronounced al-bāb, with the l clearly pronounced

So even though both words are written with الـ, they are not pronounced the same way.

This is a very important pronunciation rule in Arabic.