Otobüs bu kadar erken gelince çok şaşırdım.

Gelince comes from the verb gelmek (to come) plus the suffix -ince/-ınca.

In this sentence, gelince means something like:

• when it came
• once it came
• since it came

Here it connects the idea the bus came very early to the result I was very surprised.

So Otobüs bu kadar erken gelince çok şaşırdım literally works like:

• The bus arriving this early, I got very surprised
• more natural English: I was very surprised when the bus came this early

This -ince form is often called a converb or adverbial verb form. It links one action to another without making a full separate clause with because, when, etc.

Because Turkish often leaves out subject pronouns when they are already clear from the verb ending.

The verb şaşırdım breaks down as:

• şaşır- = be surprised
• -dı- = past tense
• -m = I

So şaşırdım already means I was surprised or I got surprised.

That is why Turkish does not need ben here. You could say Ben çok şaşırdım, but it usually sounds unnecessary unless you want emphasis, contrast, or clarity.

Here otobüs is the subject of the subordinate idea otobüs ... gelince = when the bus came.

Subjects in Turkish normally appear with no case ending in basic sentences. So:

• otobüs = the bus
• otobüsü would be the object form

In this sentence, the bus is the thing doing the action of coming, so plain otobüs is correct.

Bu kadar means this much, this much/this many, or in this context this, so, this much so.

With erken (early), it means:

• bu kadar erken = this early
• also naturally: so early

So the speaker is reacting not just to the bus arriving early, but to it arriving surprisingly early.

In Turkish, many adjectives can also function as adverbs without changing form.

So:

• erken otobüs = an early bus (adjective use)
• erken geldi = it came early (adverb use)

English often changes structure, but Turkish usually does not need a separate -ly type ending here.

Çok şaşırdım means I was very surprised or I got very surprised.

Breakdown:

• çok = very / a lot
• şaşırmak = to be surprised, to get confused, to be astonished depending on context
• şaşırdım = I was surprised / I got surprised

A useful point for English speakers: Turkish often uses a verb where English might prefer an adjective:

• Turkish: şaşırdım
• English: I was surprised

Turkish usually puts the main verb at the end, and subordinate clauses often come before the main clause.

So the structure is:

• Otobüs bu kadar erken gelince = when the bus came this early
• çok şaşırdım = I was very surprised

A more word-for-word order is:

• Bus this much early when-came, very surprised-I-was

This is normal in Turkish. English tends to put the subject and finite verb earlier, while Turkish often saves the main verb for the end.

Yes, you can, and it is very similar in meaning.

Compare:

• gelince
• geldiğinde

Both can mean when it came.

The difference is often more about style and nuance:

• gelince is very common, compact, and natural in everyday speech
• geldiğinde can sound a bit more explicit or slightly more formal/literary depending on context

In many situations, both are possible. For a learner, gelince is a very useful and common pattern to know.

Gelince does not behave like a normal fully marked tense form such as geldi (it came) or gelecek (it will come).

Instead, -ince creates a subordinate time/circumstance clause. The exact time is understood from context and from the main verb.

Here the main verb is şaşırdım (I was surprised) in the past, so the whole event is understood as past:

• When the bus came this early, I was very surprised

So gelince itself is not simply a normal past tense form, even though the English translation here uses past time.

This is due to vowel harmony.

The suffix appears in forms like:

• -ınca
• -ince
• -unca
• -ünce

The vowel changes to match the preceding vowel in the verb stem.

Since gel- contains e, the correct form is gelince.

Examples:

• bakınca = when looking
• gelince = when coming / when it came
• bulunca = when finding
• gülünce = when laughing

It mainly means when, but in real use it can also carry a cause/result feeling depending on context.

In this sentence:

• Otobüs bu kadar erken gelince çok şaşırdım

the meaning is both:

• When the bus came this early, I was very surprised
• and naturally also Because the bus came this early, I was very surprised

So the Turkish form leaves room for both a time connection and a reason connection. English often forces you to choose one wording, but Turkish can be a little broader here.

Here şaşırmak is used intransitively and means to be surprised.

So:

• şaşırdım = I was surprised / I got surprised

If you want something surprised me, Turkish often uses the causative:

• Bu beni şaşırttı = This surprised me

So the difference is:

• şaşırdım = I was surprised
• şaşırttı = it surprised someone

In your sentence, the speaker is describing their own reaction, so şaşırdım is the natural choice.

Yes, Turkish word order is flexible, though some orders sound more neutral than others.

The given sentence:

• Otobüs bu kadar erken gelince çok şaşırdım.

is very natural and neutral.

You might also hear:

• Çok şaşırdım, otobüs bu kadar erken gelince.

This can sound more conversational or dramatic, but the original version is the more standard structure for learners to copy.

In general, Turkish changes word order for:

• emphasis
• topic/focus
• style

But for everyday learning, the original sentence is an excellent model.