Başkan toplantıyı açtı.

Because it is the direct object of the verb, and it is a specific/definite one: the meeting.

In Turkish, a definite direct object usually takes the accusative ending.
The basic noun is toplantı = meeting.

So:

• toplantı = a meeting / meeting
• toplantıyı = the meeting (as a direct object)

The -y- is just a buffer consonant. Since toplantı already ends in a vowel, Turkish inserts y before the accusative ending:

• toplantı + -(y)ı → toplantıyı

The accusative suffix changes according to vowel harmony. Its pattern is:

• -ı
• -i
• -u
• -ü

After a vowel, it becomes:

• -yı
• -yi
• -yu
• -yü

Since the last vowel in toplantı is ı, the correct form is -(y)ı:

• toplantı → toplantıyı

Literally, açtı means opened.
In this sentence, it means opened the meeting or more naturally in English, started the meeting.

This is a normal Turkish expression:

• toplantıyı açmak = to open/start the meeting

So even though the basic meaning of açmak is to open, it is also used for ceremonies, meetings, sessions, etc.

It comes from the verb stem aç- (to open) plus the simple past tense ending.

Structure:

• aç- = open
• -tı = past tense

So:

• açtı = opened

The past tense suffix is often taught as -DI, but it changes depending on vowel harmony and consonant voicing:

• aç + DI → açtı

It becomes t instead of d because ç is a voiceless consonant.

Because the most neutral Turkish word order is Subject – Object – Verb.

So:

• Başkan = subject
• toplantıyı = object
• açtı = verb

That gives:

• Başkan toplantıyı açtı.

Turkish word order can change for emphasis, but putting the verb at the end is the standard, neutral pattern.

Sometimes, yes — but not always.

Başkan is a broad word that can mean:

• president
• chairperson / chair
• head
• sometimes mayor, depending on context

In Başkan toplantıyı açtı, the most natural English translation is often The chairperson opened the meeting or The president opened the meeting, depending on the situation.

So the exact meaning depends on context.

Turkish does not have definite articles like English the.

Instead, definiteness is often understood through:

• context
• word order
• case marking, especially the accusative for direct objects

That is why toplantıyı can carry the meaning of the meeting without a separate word for the.

Yes, it would sound different.

• Başkan toplantıyı açtı = The chair/president opened the meeting
  
  • specific meeting
  • definite object

Without the accusative:

• Başkan toplantı açtı

this would not be the normal way to say opened the meeting. It would sound more indefinite or unnatural in this context. With açmak, when you mean a particular meeting was opened, Turkish normally uses the accusative: toplantıyı.

Not necessarily.

Başkan is generally gender-neutral in modern Turkish. It can refer to:

• a man
• a woman

So it can mean chairperson, president, or head, without specifying gender.

In this sentence, it is capitalized because it is the first word of the sentence.

If it appeared in the middle of a sentence as a common noun, it would usually be lowercase:

• Başkan toplantıyı açtı.
• Toplantıyı başkan açtı.

Capitalization does not by itself mean it is a proper name here.

A rough pronunciation guide is:

• Başkan ≈ BUSH-kahn
• toplantıyı ≈ top-lahn-tuh-yuh
• açtı ≈ AHCH-tuh

A few useful points:

• ş = sh
• ç = ch
• ı is a vowel that does not exist in English; it is like a relaxed, unrounded vowel, somewhat like the second vowel in sofa, but farther back

So the whole sentence is approximately:

• BUSH-kahn top-lahn-tuh-yuh AHCH-tuh

Yes. Turkish allows different word orders for emphasis.

For example:

• Başkan toplantıyı açtı. = neutral
• Toplantıyı başkan açtı. = emphasizes it was the chair/president who opened the meeting
• Açtı başkan toplantıyı. = possible in special contexts, but much less neutral

The default sentence you have is the most straightforward and standard one.