Learn Farsi: اشغال قلمرو یک اشتباه بزرگ بود. - The occupation of the territory was a big mistake.

It uses the root consonants sh-gh-l (related to work or being busy, like in the word شغل). By plugging these into the 'e-C1-C2-â-C3' pattern, we get e-sh-gh-â-l (اشغال). This pattern often denotes causing a state or performing an action, so here it translates to the action of 'occupying' or 'occupation'.

The word اشتباه (eshtebâh) also starts with 'e' and has an 'â' near the end. Is it also an Ef'âl noun?

No, it actually belongs to a different Arabic pattern (Form VIII, or Efte'âl). The root consonants are sh-b-h (related to doubt or resemblance). The 't' in the middle of اشتباه is a structural part of that specific pattern, not a root letter. While it looks similar to the Ef'âl pattern, spotting the 't' after the first root letter helps you tell them apart.

What is the literal meaning behind the word قلمرو (qalamro)?

It translates literally to 'pen-going'. It is a compound of قلم (qalam, meaning pen) and رو (ro, the present stem of رفتن, to go). Historically, it referred to the geographic area where a ruler's written decrees (the pen) reached and were enforced, which is how it came to mean 'territory' or 'realm'.

Why isn't there an ezâfe linking قلمرو (qalamro) to یک (yek)?

Because اشغال قلمرو (The occupation of the territory) is the complete subject of the sentence, and یک اشتباه بزرگ بود (was a big mistake) is the predicate. The ezâfe connects words within a phrase (like a noun and its possessor), but you never use it to link the subject to the rest of the sentence.