Learn Hardcore Turkish: Bahar gelince ağaçlar çiçek açıyor.

The suffix -ince attaches directly to a verb stem to form a time-clause meaning “when,” “once,” or “as soon as.”
• gelmek = to come
• gelmek + -ince = gelince = when (it) comes / as soon as (it) comes

Why is gelince written as one word instead of two?

Turkish is agglutinative, so you attach most grammatical markers (like tense, mood, or conjunctional suffixes) directly onto the root without spaces or hyphens.

How do I know whether to use -ince, -ınca, -ünce, or -unca?

You pick the suffix vowel by vowel harmony—match the last vowel of the root:
• Root vowel e or i → -ince
• Root vowel a or ı → -ınca
• Root vowel ö or ü → -ünce
• Root vowel o or u → -unca
Since gelmek ends in e, you use -ince.

Why doesn’t açıyor change when the subject ağaçlar is plural?

In Turkish the 3rd-person present (progressive) form is unmarked for number. You add -yor + person ending:
• ben açıyorum (I am)
• sen açıyorsun (you are)
• o açıyor (he/she/it is)
• (biz, siz, onlar) açıyor (we/you/they are)
So açıyor works for both singular and plural 3rd persons; the subject’s plurality comes from ağaçlar.

Why doesn’t çiçek take an accusative suffix in ağaçlar çiçek açıyor?

Only definite, specific objects get the accusative -ı/-i/-u/-ü. Here çiçek is indefinite/general—trees bloom flowers in general—so it stays unmarked.

What tense and aspect does açıyor express here?

It’s the present continuous (progressive) tense. In contexts like this it often describes a habitual or natural process:
“Whenever spring comes, trees are (continuously) blossoming.”

Can I replace gelince with geldiğinde? What’s the nuance?

Yes. geldiğinde comes from the verbal noun plus locative (-diği + -nde) and also means “when it has come.”
• bahar gelince = when spring comes (more colloquial, direct)
• bahar geldiğinde = when spring has come (a bit more formal or explicit)
In most everyday cases they’re interchangeable.

Why is bahar in the nominative case and not baharda?

Here bahar is the subject of the subordinate clause “when spring comes,” so it stays in nominative. baharda would be the locative case meaning “in spring” as a time adverbial, not “when spring arrives.”

Is the word order fixed? Could I say Ağaçlar bahar gelince çiçek açıyor?

Turkish word order is flexible thanks to case and verb markers. The default is S O V, but time clauses often come first. You can say:
• Bahar gelince ağaçlar çiçek açıyor.
• Ağaçlar bahar gelince çiçek açıyor.
Both mean the same; fronting an element just shifts the emphasis.