Onun boyu çok uzun.

Yes. Onun can mean his, her, or its, depending on context.

Turkish does not mark grammatical gender the way English does, so the same word is used for:

• his
• her
• its

In this sentence, onun is the possessor, so onun boyu means his/her height or his/her stature.

Because boyu includes a possessive suffix.

• boy = height, stature, length
• boyu = his/her/its height

In Turkish, possessed nouns usually take a possessive ending. So:

• onun boyu = his/her height
• literally: of him/her + height-his/hers

This is a very common Turkish structure:

• benim adım = my name
• senin evin = your house
• onun boyu = his/her height

So boyu is not a different dictionary word; it is boy with the 3rd person possessive ending.

Because in Turkish, the verb to be is often not expressed in the present tense, especially in the 3rd person.

So:

• Onun boyu çok uzun. = His/Her height is very great / He or she is very tall.

The idea of is is understood automatically.

This is normal in Turkish:

• Ev büyük. = The house is big.
• O hasta. = He/She is sick.
• Onun boyu çok uzun. = His/Her height is very tall / He or she is very tall.

You may also sometimes see uzundur, which adds a more formal, definite, or explanatory tone:

• Onun boyu çok uzundur.

But the version without it is completely normal.

It can mean long or tall, depending on context.

• For objects, uzun often means long
• For people, it often means tall

In this sentence, because we have boyu meaning height/stature, uzun is understood as tall.

So:

• Onun boyu çok uzun. = He/She is very tall.
• literally, it is closer to His/Her height is very great/long

This is a good example of how Turkish often expresses tall through the idea of height.

Yes, a little.

A very natural English translation is:

• He/She is very tall.

But the Turkish structure is more literally:

• His/Her height is very tall/long or more naturally in English:
• His/Her height is considerable
• He/She has a great height
• He/She is tall

Turkish often uses body-part or measurement nouns in ways that sound slightly more literal than English.

Yes, often you can.

• O çok uzun. = He/She/It is very tall/long.
• Onun boyu çok uzun. = His/Her height is very tall; He/She is very tall.

The difference is mainly one of focus:

• O çok uzun focuses directly on the person or thing
• Onun boyu çok uzun focuses specifically on the person’s height

So if you are talking about a person, both can work, but onun boyu çok uzun emphasizes height more explicitly.

It can often be omitted if the context is already clear.

So you might hear:

• Boyu çok uzun. = His/Her height is very great; He/She is very tall.

Turkish frequently drops pronouns and other words when the meaning is obvious from context.

Including onun can:

• make the reference clearer
• add emphasis
• create contrast, like His height is very tall, but hers isn’t

So:

• Boyu çok uzun. = natural if we already know who is being discussed
• Onun boyu çok uzun. = clearer or more emphatic

Because Turkish usually puts the predicate at the end of the sentence.

Here the parts are:

• Onun boyu = his/her height
• çok = very
• uzun = tall/long

So the sentence builds toward the final description:

• His/Her height
  • very
    • tall

This is normal Turkish word order.

Also, çok comes before the adjective it modifies:

• çok uzun = very tall/very long
• çok güzel = very beautiful
• çok büyük = very big

Not always.

Boy can refer to:

• height or stature for a person
• length or size in some other contexts

In this sentence, it clearly means height.

So:

• onun boyu = his/her height
• not his/her age
• not his/her son or anything like that

A learner should remember that boy is a measurement-related word, and the exact English translation depends on context.

It is pronounced roughly like bo-yu, in two syllables:

• bo
• yu

A few helpful notes:

• the oy in boy sounds somewhat like the oy in English boy
• the final u is a Turkish u, not exactly the same as English oo, but somewhat similar
• boyu flows smoothly, almost like boyoo, but still with two syllables

And:

• onun is roughly o-nun
• çok is roughly chohk
• uzun is roughly oo-zun

Yes, especially if onun is included.

For example, it could imply contrast:

• Onun boyu çok uzun, kardeşinin değil. = He/She is very tall, but his/her sibling isn’t.

So using onun may suggest:

• that person’s height in particular
• not someone else’s
• or a clearer topic shift in conversation

Without special context, though, the sentence is simply a normal statement meaning He/She is very tall.