İlkbahar şenliği başlayınca herkes parkta toplandı.

It means when it began/started. In context, it can also carry a slight once/as soon as feeling.

So:

• İlkbahar şenliği başlayınca = when the spring festival began
• herkes parkta toplandı = everyone gathered in the park

The ending -(y)ınca / -(y)ince is a very common way to make a time clause in Turkish.

It is formed from the verb başlamak (to begin/start).

Step by step:

• dictionary form: başlamak
• remove -mak → başla-
• add the suffix -(y)ınca
• because başla- ends in a vowel, Turkish inserts a buffer y
• result: başlayınca

So:

• başla + yınca → başlayınca

The suffix appears as -ınca here because of vowel harmony. With front vowels, you would often see -ince instead.

Because -ınca is not a normal tense ending. It creates an adverbial clause meaning when/once.

Turkish often lets the main verb show the overall time of the sentence:

• toplandı = past
• so the whole sentence is understood in the past

That is why English naturally translates it as when it began.

For example:

• Festival başlayınca herkes geldi. = When the festival started, everyone came.
• Festival başlayınca geleceğiz. = When the festival starts, we will come.

So başlayınca itself does not force past time; the context and main verb do that.

Yes, İlkbahar şenliği başladığında herkes parkta toplandı is also possible.

The difference is mostly one of style and nuance:

• başlayınca is very common, natural, and often feels a bit more direct: when / once it started
• başladığında also means when it started, but can sound a little more formal, written, or neutral

In many everyday contexts, both work.

Because this is a common Turkish noun compound.

In Turkish, when one noun modifies another noun, the second noun usually takes the compound ending -(s)I:

• ilkbahar = spring
• şenlik = festival
• ilkbahar şenliği = spring festival

So the second word changes:

• şenlik + i → şenliği

Also note the sound change:

• final k often softens before a vowel
• so şenlik becomes şenliğ- before -i
• that gives şenliği

This is not a literal possession like the festival of spring in a strict English sense; it is just the normal Turkish way to form this kind of compound noun.

Yes. In İlkbahar şenliği başlayınca, the thing that begins is ilkbahar şenliği.

So the sentence has two different subjects in its two parts:

• subordinate clause: İlkbahar şenliği başlayınca → subject = ilkbahar şenliği
• main clause: herkes parkta toplandı → subject = herkes

This is completely normal in Turkish. A -ınca clause does not have to share the same subject as the main clause.

Because herkes means everyone, and in standard Turkish it is grammatically singular.

So the normal pattern is:

• Herkes toplandı. = Everyone gathered.

This is similar to English, where everyone is also grammatically singular, even though it refers to many people.

You may hear plural agreement in casual speech sometimes, but the standard form is herkes toplandı.

Because parkta means in the park / at the park, while parka means to the park.

Here the sentence is describing where everyone gathered, not the direction of movement:

• parkta toplandı = gathered in/at the park
• parka gitti = went to the park

Also, the locative suffix is -da / -de / -ta / -te. Here it becomes -ta because park ends in the voiceless consonant k:

• park + ta → parkta

Not in the way an English learner might first think.

Here toplandı comes from toplanmak, which means to gather / assemble. With people, it is a normal intransitive verb:

• Herkes toplandı. = Everyone gathered.

It is related to:

• toplamak = to collect, gather something

But toplanmak has become the usual verb for coming together. So in this sentence, it does not mean something awkward like everyone was collected. It simply means everyone gathered.

Because Turkish very often puts background information first, especially time expressions and time clauses.

So this order is very natural:

• İlkbahar şenliği başlayınca → first set the time
• herkes parkta toplandı → then tell the main event

It is a smooth, standard Turkish sentence pattern.

Turkish word order is flexible, so other arrangements are possible, but this version sounds very natural because it introduces the situation before the main action.