Learn Farsi: سگ پستاندار است و حس بویایی بسیار قوی دارد. - The dog is a mammal and has a very strong sense of smell.

This is a common way to make generic statements in Farsi. You use a bare singular noun without any plural or definite/indefinite markers, paired with a singular verb like است. This represents the entire species, just like saying 'The dog' or 'Dogs' in English.

Since the sentence means 'dogs are mammals,' why isn't پستاندار plural?

Because the subject سگ is grammatically singular to represent the category, the noun describing it, پستاندار, must also be singular. They match grammatically to keep the generic reference consistent.

Where do the Ezāfe connections go in the phrase حس بویایی بسیار قوی?

It reads as hes-e buyāyi-ye besyār qavi. The first Ezāfe links the noun حس (sense) to بویایی (smelling). Because بویایی ends in a vowel, it takes a 'ye' sound to link it to the adjective phrase بسیار قوی (very strong).

What is the literal breakdown of the word پستاندار?

It is a compound word made of پستان (breast or udder) and the present stem suffix دار (from داشتن, meaning to have or hold). So in Farsi, a mammal is literally a 'breast-possessor'.