Projeyi aylardır aynı tempoda sürdürdük; artık sadece ince ayar kaldı.

It’s the accusative case marker showing a specific direct object.

• Projeyi = proje + y + i (buffer y
  • accusative -i by vowel harmony).
• You use accusative when the object is definite/specific.
• Contrast:
  • Projeyi sürdürdük = We kept the project going (that particular project).
  • Bir proje sürdürdük = We kept a project going (non-specific).

Yes, but the structure changes because the verbs differ:

• sürdürmek is transitive: Projeyi sürdürdük (we sustained/maintained it).
• devam etmek is intransitive and takes dative: Projeye devam ettik (we continued with the project).
• Projeyi devam ettik is ungrammatical. Nuance: sürdürmek emphasizes actively keeping something going; devam etmek emphasizes that something continues.

Sürdürdük (we maintained) frames that phase as completed; it fits naturally before artık and kaldı (now only X is left). Alternatives:

• Sürdürüyoruz = we are still maintaining it now (use if the same pace continues into the present).
• Sürdürüyorduk = we were maintaining it (past progressive, useful in a narrative about the past).

That’s not idiomatic. For durations you typically use:

• aylardır (for months now)
• or aylarca (for months). Plain plural aylar doesn’t by itself mean “for months.”

Artık means “now/at this point/by now” in affirmative contexts. With negatives it often means “no longer/anymore.”

• Affirmative: Artık sadece ince ayar kaldı = now only fine-tuning is left.
• Negative example: Artık vaktimiz yok = we no longer have time.

Sadece focuses “only/just” on what follows. It’s placed immediately before the focused element: sadece ince ayar. Common alternatives:

• yalnızca (synonym, slightly more formal)
• yalnız (can mean “only,” but also “however,” so it can be ambiguous)
• sırf (colloquial “just/merely,” sometimes implies “for the sake of”)

It’s a common collocation meaning “fine-tuning/fine adjustment,” used literally (e.g., calibration) and metaphorically (finishing touches). Related expressions:

• ince ayar yapmak = to fine‑tune
• son rötuşlar = final touches (more colloquial/visual)

Turkish often uses simple past (-dı) to express a change of state whose result holds now. Kaldı conveys “has now become the case that only X remains.” Variants:

• kalmış = apparently/it seems only X is left (evidential, report/inference)
• kalıyor = is remaining (progressive; less natural for a snapshot result)

A semicolon neatly links two closely related clauses. You could also use:

• A period: … sürdürdük. Artık sadece …
• A comma plus a linker: … sürdürdük, bu yüzden/ve artık sadece … A bare comma is common informally but less ideal in careful writing.

Turkish is flexible; word order shifts focus.

• Given: Projeyi aylardır aynı tempoda sürdürdük foregrounds the project.
• Aylardır projeyi aynı tempoda sürdürdük highlights the duration.
• Projeyi aynı tempoda aylardır sürdürdük is possible but less natural; time expressions usually come earlier. The verb typically comes last.

Because proje ends in a vowel. Turkish inserts the buffer consonant y before vowel-initial suffixes:

• proje + i → projeyi (accusative) Vowel harmony picks -i (last vowel e is front-unrounded).