Kardeşim ortaokula başladı.

Because -im here is a 1st person singular possessive suffix, meaning my.

• kardeş = sibling
• kardeş-im = my sibling

So Turkish often expresses possession by adding a suffix to the noun instead of using a separate word like my.

It can mean either. Kardeş is gender-neutral.

So kardeşim can mean:

• my brother
• my sister
• more literally, my sibling

Context tells you which one is meant.

Also, in everyday Turkish, kardeş often refers especially to a younger sibling, while older siblings are commonly called:

• ağabey / abi = older brother
• abla = older sister

But in a broad sense, kardeş can still mean sibling in general.

Because başlamak normally takes the dative case in Turkish when you say what someone starts.

So:

• ortaokul = middle school
• ortaokul-a = to middle school

That gives:

• ortaokula başlamak = to start middle school

This is different from English, where start usually takes a direct object. In Turkish, with başlamak, the thing started is often marked with the dative.

Because of vowel harmony.

The dative suffix has two forms:

• -a
• -e

Which one you use depends on the last vowel of the word.

In ortaokul, the last vowel is u, which is a back vowel, so the correct form is -a:

• ortaokul + a = ortaokula

If the last vowel were a front vowel, you would use -e instead.

Başladı is the simple past tense, 3rd person singular form of başlamak.

Breakdown:

• başla- = start, begin
• -dı = past tense suffix

So başladı means started / began.

The -dı part changes form according to vowel harmony and consonant rules, so you may also see forms like -di, -du, -dü, -tı, -ti, -tu, -tü with other verbs.

Because Turkish usually does not need subject pronouns when the subject is already clear.

Here, the subject is explicitly given as kardeşim. So adding o would be unnecessary:

• Kardeşim ortaokula başladı. = natural
• O kardeşim ortaokula başladı. = possible only in a special context, but not the normal neutral sentence

Also, Turkish o can mean he, she, or it, so gender is not marked in the pronoun either.

Because Turkish is typically a verb-final language.

A neutral Turkish sentence often follows this pattern:

• subject + other elements + verb

So here:

• Kardeşim = subject
• ortaokula = complement
• başladı = verb

That makes the sentence feel very normal and natural in Turkish.

Yes, ortaokul is written as one word.

It is made from:

• orta = middle
• okul = school

So it literally means middle school.

When you add the dative ending, it becomes:

• ortaokul + a = ortaokula

Because Turkish does not have articles like English a/an and the.

So a noun can often stand alone:

• okul = school / the school / a school

Context tells you which meaning is intended.

If Turkish wants to emphasize a/an, it can use bir, but that is not needed here.

In standard Turkish, no. That sounds wrong with başlamak.

Why?

• ortaokulu is the accusative form
• but başlamak normally takes the dative, not the accusative

So the correct form is:

• Kardeşim ortaokula başladı.

Not:

• Kardeşim ortaokulu başladı.

This is one of those verb patterns you simply need to learn: some Turkish verbs require a certain case, and başlamak commonly goes with the dative.

It can naturally mean starting middle school as a stage of education, not necessarily emphasizing one specific school building.

In Turkish, this kind of sentence often focuses on the new school level or institution being entered.

If you wanted to be more specific, Turkish could add more detail, such as the name of the school or another modifier. But as it stands, the sentence works well for the general idea of starting middle school.