Learn Farsi: من اعتبار ویزایم را بررسی نکرده بودم. - I had not checked the validity of my visa.

For a compound verb, the first word (بررسی) stays exactly the same. You build the negative past perfect on the second verb (کردن). You add the negative prefix نـ (na-) to its past participle to get نکرده (nakarde), and follow it with the simple past of بودن (budan) for 'I', which is بودم (budam). Together, this makes بررسی نکرده بودم (I had not checked).

Why is there an extra ی (y) in the word ویزایم (my visa)?

The base word is ویزا (vizâ), which ends in the long vowel 'â'. When you add the possessive ending ـَم (-am) for 'my', you cannot put two vowels right next to each other. Farsi uses the consonant ی (y) as a bridge, making it ویزایم (vizâ-y-am).

How does the sentence say 'validity OF my visa'?

It uses the ezâfe grammatical connector, which is a short 'e' sound added to the end of the first word. Although it is usually not written out in the Farsi alphabet, the phrase is pronounced 'e'tebâr-e vizâyam'. This invisible '-e' links اعتبار (validity) to ویزایم (my visa).

Why do we need the word را (râ) here?

The word را (râ) marks a specific direct object. Because the thing you did not check is a specific item—the validity of your visa—you must place را immediately after the entire noun phrase to show it is the target of the verb.