إذا كنت في البيت في المساء، أغلق الباب بالمفتاح.

إذا introduces a condition. In this sentence, it means if.

A useful nuance:

• إذا often suggests a condition that is realistic, expected, or repeated.
• Because of that, it can sometimes feel close to when in English.

So here the sentence can mean something like:

• If you are at home in the evening, lock the door with the key
• or, in some contexts, When you are at home in the evening, lock the door with the key

This is a very common question.

كنت is formally the past form of كان for you masculine singular: you were. But after إذا, Arabic often uses a past-form verb even when the meaning is present or future in English.

So:

• إذا كنت في البيت = if you are at home / if you happen to be at home

The form looks past, but the whole conditional structure gives it a non-past meaning here.

The sentence is addressing one male person.

That is why you have:

• كنت = you were/are for one masculine person
• أغلق = the command form addressed to one masculine person

If you were speaking to a woman, you would say:

• إذا كنتِ في البيت في المساء، أغلقي الباب بالمفتاح.

If you were speaking to a group of men or a mixed group:

• إذا كنتم في البيت في المساء، أغلقوا الباب بالمفتاح.

Because Arabic uses في for both place and time expressions.

Here:

• في البيت = in the house / at home
• في المساء = in the evening

So the first في gives location, and the second في gives time.

Because Arabic verb forms usually already contain the subject.

So أغلق by itself already means:

• close! / lock! addressed to one male person

Arabic does have separate pronouns such as أنت, but they are usually omitted unless needed for emphasis or contrast.

So Arabic normally says:

• أغلق الباب

not:

• أنت أغلق الباب

unless there is a special reason to emphasize you.

By itself, أغلق usually means close or shut.

But in this sentence, because of بالمفتاح = with the key, the natural meaning is lock:

• أغلق الباب = close/shut the door
• أغلق الباب بالمفتاح = lock the door with the key

In modern usage, أقفل is also very common for close/lock, depending on region and context.

بالمفتاح is made of:

• بـ = with / by means of
• المفتاح = the key

So بالمفتاح literally means with the key or using the key.

It is written as one word because بـ attaches directly to the noun. Also:

• بـ + ال becomes بال

So:

• ب + المفتاح → بالمفتاح

Because in Arabic, في البيت is a very common way to express the idea of being at home.

Literally, it is in the house, but very often the natural English translation is simply at home.

So depending on context:

• في البيت can be in the house
• or more naturally at home

In full vocalization, you could write it like this:

إِذا كُنْتَ في البَيْتِ في المَساءِ، أَغْلِقِ البابَ بالمِفْتاحِ.

The important endings are:

• البيتِ: genitive, because it comes after في
• المساءِ: genitive, because it comes after في
• البابَ: accusative, because it is the direct object of أغلق
• المفتاحِ: genitive, because it comes after بـ

A small pronunciation note:

• The basic command form is أَغْلِقْ
• Before الـ in connected speech, you often get أَغْلِقِ البابَ to avoid two consonants meeting awkwardly

Yes.

In this sentence, the ل of ال is pronounced in:

• البيت
• المساء
• الباب
• المفتاح

That is because these words begin with moon letters, not sun letters.

So you pronounce:

• al-bayt
• al-masāʾ
• al-bāb
• al-miftāḥ

not an assimilated form.

Yes, sometimes.

Because إذا often introduces a real or expected situation, English may translate it as either if or when, depending on context.

So this sentence could be understood as:

• If you are at home in the evening, lock the door with the key
• or
• When you are at home in the evening, lock the door with the key

If you want the most basic beginner translation, if is perfectly fine.