يترك صديقي الكتاب في البيت.

Because Modern Standard Arabic often uses verb–subject–object order.

So:

• يترك = the verb
• صديقي = the subject
• الكتاب = the object

This is normal Arabic word order.
You can also say صديقي يترك الكتاب في البيت, with the subject first, but starting with the verb is very common in MSA.

يترك is in the imperfect form, which often corresponds to the English present.

Depending on context, it can mean things like:

• he leaves
• he is leaving
• sometimes even he will leave

Arabic does not always separate simple present and present progressive the way English does. Context tells you which meaning is intended.

The verb يترك comes from the root ت-ر-ك and its basic meaning is to leave.

Depending on context, it can also carry ideas like:

• leave behind
• put down and go away
• abandon

In this sentence, it has the ordinary sense of leaving something somewhere.

The -ي is the suffix meaning my.

So:

• صديق = friend
• صديقي = my friend

Arabic often expresses possession by attaching a pronoun directly to the noun, instead of using a separate word like my.

Because possessed nouns normally do not take الـ.

So Arabic says:

• صديقي = my friend

not something like:

• الصديقي

The suffix -ي already makes the noun definite, because my friend is automatically a specific person.

Because both nouns are definite here.

• الكتاب = the book
• البيت = the house / the home

Arabic uses الـ for the.
Also, Arabic often says في البيت where English might simply say at home, so the definite article can appear in places where English would not necessarily use the.

It can mean either, depending on context.

• بيت can mean house
• It can also mean home

So في البيت may be understood as:

• in the house
• at home

Both are natural translations, and the wider context decides which one sounds best.

There are two main clues:

1. Word order
   In a normal verb-first sentence, the noun right after the verb is often the subject, and the next noun is often the object.

2. Case, in fully vocalized Arabic
   If the sentence were fully vowelled, الكتاب would appear as الكتابَ, with the accusative ending that marks the direct object.

So Arabic grammar identifies الكتاب as the thing being left.

A fully vocalized version is:

يَتْرُكُ صَدِيقِي الكِتَابَ فِي البَيْتِ

This shows:

• يَتْرُكُ = verb
• صَدِيقِي = subject, my friend
• الكِتَابَ = object, marked accusative
• فِي البَيْتِ = prepositional phrase, with البيتِ in the genitive after في

One important detail: the subject صديقي is nominative in function, but its case ending is not clearly shown because of the attached -ي.

A helpful transliteration is:

yatruku ṣadīqī al-kitāba fī al-bayti

A few pronunciation notes:

• ṣ in ṣadīqī is an emphatic s
• ā is a long a
• ī is a long ee
• al- is pronounced clearly in الكتاب and البيت, because k and b are not sun letters

Yes. For example:

صديقي يترك الكتاب في البيت

This still means the same basic thing, but it puts more focus on صديقي as the topic.

So:

• يترك صديقي الكتاب في البيت = neutral, very standard verb-first style
• صديقي يترك الكتاب في البيت = more subject-focused

Both are correct MSA.