Yetmiş kişi toplantıya katıldı.

• Yetmiş = seventy
• kişi = person / people
• toplantıya = to the meeting
• katıldı = attended / joined / participated

So the sentence means something like Seventy people attended the meeting.

In Turkish, when a noun comes after a number, it usually stays singular.

So:

• yetmiş kişi = seventy people
• not yetmiş kişiler

This is different from English, where we normally say seventy people with a plural noun. In Turkish, the number already shows plurality, so the noun does not need a plural ending.

Kişi means person and is often used in counting or formal contexts.

For example:

• üç kişi = three people
• elli kişi geldi = fifty people came

İnsan also means person / human, but it is more general and less specifically a counting word.

In this sentence, yetmiş kişi is the natural way to say seventy people.

Because the verb katılmak usually takes the dative case, which often corresponds to to in English.

• toplantı = meeting
• toplantıya = to the meeting

The ending here is the dative ending:

• -(y)a / -(y)e

Since toplantı ends in a vowel, Turkish adds a buffer consonant y:

• toplantı + ya → toplantıya

So toplantıya katılmak means to attend / join the meeting.

Because katılmak does not take a direct object in Turkish. It takes a noun in the dative case.

So:

• toplantıya katılmak = to attend the meeting
• literally closer to to join to the meeting

Using the accusative (-ı / -i / -u / -ü) would be ungrammatical here.

This is something you just learn with the verb:

• derse katılmak = to attend class
• partiye katılmak = to attend the party
• toplantıya katılmak = to attend the meeting

Katıldı is in the simple past tense.

It is built like this:

• katıl- = verb stem (join / attend)
• -dı / -di / -du / -dü = past tense ending
• here it becomes -dı because of vowel harmony

So:

• katıldı = he/she/it attended or attended

In Turkish, third-person singular often also works for plural subjects when the subject is stated:

• Yetmiş kişi katıldı = Seventy people attended

In Turkish, when the subject is a plural group, the verb is often still in third-person singular, especially when the subject is clearly stated.

So this is perfectly normal:

• Yetmiş kişi toplantıya katıldı

You may sometimes see plural verb forms in some contexts, but with counted groups like this, singular verb agreement is very common and natural.

The dictionary form is katılmak.

This is the infinitive, meaning:

• to join
• to attend
• to participate

Examples:

• Toplantıya katılmak istiyorum. = I want to attend the meeting.
• Derse katıldı. = He/She attended the class.

Not always. It depends on context.

Katılmak can mean:

• to attend
• to join
• to participate in
• sometimes to agree with, when used with a different structure

Examples:

• Toplantıya katıldı. = He/She attended the meeting.
• Yarışmaya katıldı. = He/She participated in the competition.
• Sana katılıyorum. = I agree with you.

So in this sentence, attended is the best translation.

Yes. Turkish word order is flexible, but the most neutral order is often:

Subject + object/complement + verb

So:

• Yetmiş kişi = subject
• toplantıya = complement
• katıldı = verb

This gives the neutral meaning:

• Seventy people attended the meeting.

Other orders are possible for emphasis, for example:

• Toplantıya yetmiş kişi katıldı.
  This puts more focus on the meeting or on the fact that seventy people attended it.

The verb often comes at the end in standard Turkish.

A rough pronunciation guide:

• Yetmiş → YET-mish
• kişi → kee-SHUH (the final vowel is the Turkish ı, not English ee)
• toplantıya → top-lan-TUH-yah
• katıldı → kah-tuhl-DUH

A few pronunciation notes:

• ş sounds like sh
• ı is a vowel that does not exist in English; it is a back, unrounded vowel
• stress in Turkish is often toward the end, but pronunciation can vary by word

It gives a straightforward past event: the attendance happened and is viewed as completed.

So katıldı means:

• attended
• joined
• participated

It is not like was attending or has attended specifically. Depending on context, English may translate it differently, but the basic idea is a completed past action.

No, that would sound unnatural.

For counted people, Turkish normally uses kişi:

• yetmiş kişi

Using insanlar after a number is not the normal pattern here. Also, after numbers, Turkish nouns usually stay singular, so insanlar would already be problematic.

The natural sentence is:

• Yetmiş kişi toplantıya katıldı.

You can make it negative by adding the negative marker to the verb:

• Yetmiş kişi toplantıya katılmadı. = Seventy people did not attend the meeting.

Breakdown:

• katıl- = attend/join
• -ma- = negation
• -dı = past tense

So:

• katıldı = attended
• katılmadı = did not attend

A simple yes/no question would be:

• Yetmiş kişi toplantıya katıldı mı? = Did seventy people attend the meeting?

The question particle is mı / mi / mu / mü, chosen by vowel harmony, and it is written separately.

So:

• katıldı = attended
• katıldı mı? = did ... attend?

Here are some useful parallels:

• On kişi derse katıldı. = Ten people attended the class.
• Yüz kişi seminere katıldı. = A hundred people attended the seminar.
• Beş öğrenci yarışmaya katıldı. = Five students participated in the competition.
• Otuz çalışan toplantıya katıldı. = Thirty employees attended the meeting.

These all follow the same pattern:

• number + singular noun
• noun in dative case
• katılmak in the past tense