Learn Farsi: من هرگز چنین ایده‌ی مضحکی نشنیده‌ام. - I have never heard such a ridiculous idea.

chenin (such / this kind of) belongs to a special group of demonstrative adjectives that always come before the noun. Unlike standard descriptive adjectives that follow the noun, pre-nominal adjectives like chenin and chenân directly precede the noun without using an Ezafe to link them.

How does the sentence say "such a ridiculous idea"? Where is the word for "a"?

The "a" is represented by the indefinite suffix -i at the very end of the noun phrase, on the word mozhheki (ridiculous). When you have a noun and an adjective linked by an Ezafe (ide-ye mozhhek), the indefinite -i attaches to the end of the entire phrase, giving us ide-ye mozhheki.

Why is there a ye (ی) written at the end of ide (ایده‌ی)?

This is the written form of the Ezafe for words ending in vowels. The word ide (idea) ends in a silent 'he' (ه), which represents an 'e' vowel sound. To connect it to the following adjective, you must add the Ezafe. Because you cannot have two vowels back-to-back, a 'y' sound is inserted to bridge them, which is written as ی.

Why use the present perfect nashnide-am instead of the simple past?

The present perfect nashnide-am (I have not heard) is used here with hargez (never) because it describes a life experience extending from the past up to the present moment. This usage aligns perfectly with how the present perfect is used for experiences in English.