Learn Farsi: این سم ضد ساس بسیار موثر است. - This anti-bedbug poison is very effective.

The word ضد means opposite or against. To form an anti-[noun] compound adjective, you use ضد followed by the ezafe vowel (an 'e' sound), then the noun. So ضد ساس is pronounced zedd-e sās, making a single adjective phrase that means anti-bedbug.

Is there a hidden ezafe between سم (poison) and ضد ساس (anti-bedbug)?

Yes! Because ضد ساس acts as an adjective describing the poison, you need an ezafe to link them. The whole noun phrase is pronounced sam-e zedd-e sās. As usual in Farsi script, this first ezafe between the noun and its compound adjective is not written out.

Why is موثر (effective) sometimes written with a small symbol over the vav (مؤثر)?

The word موثر (mo'asser) comes from Arabic and originally includes a glottal stop (a catch in the throat) represented by the hamza symbol over the و. In modern Persian, this glottal stop is often dropped in speech, so it is extremely common to write موثر without the hamza, though both spellings are perfectly correct.

Could I use خیلی (kheyli) instead of بسیار (besyār) here?

Absolutely. خیلی is the standard conversational word for very. بسیار means the exact same thing but sounds a bit more formal or written. You will often see بسیار on product labels, in commercials, or in official instructions, which perfectly fits the context of describing a commercial poison.