Learn Farsi: یک کالای گران الزاماً کیفیت بهتری از یک کالای ارزان ندارد. - An expensive product does not necessarily have better quality than a cheap product.

الزاماً (elzâman) means "necessarily". It is formed from the Arabic-derived noun الزام (elzâm - obligation or necessity) plus the suffix ـاً (-an). In Farsi, adding this suffix to certain words turns them into adverbs of manner, working very much like adding "-ly" in English.

Why is الزاماً written with that double-dash symbol over an alef?

That symbol is called a tanvin, which comes from Arabic orthography. In Farsi, the combination ـاً is always pronounced "-an". Whenever you see this symbol at the end of a word, you don't read the alef as an "â" vowel; you just read the whole suffix as "-an". You have probably seen it in other common adverbs like لطفاً (lotfan - please) or حتماً (hatman - definitely).

Why does the adjective بهتر have a ی at the end to make بهتری?

This ی (-i) is the indefinite marker, translating roughly to "a" in "a better quality". When you have a noun linked to an adjective by the ezafe vowel, such as کیفیتِ بهتر (keyfiyat-e behtar - better quality), the indefinite ی must attach to the very end of the entire phrase, which means it gets tacked onto the adjective rather than the noun itself.

How is the "than" part of the comparison expressed in this sentence?

To say "than" in Farsi comparisons, we use the preposition از (az), which usually means "from". The structure is: [comparative adjective] + + [the thing being compared to]. So, literally translates as "a better quality from a cheap product".