Learn Farsi: ما امروز میزبان باادبی داریم.

In Farsi, when a noun is described by an adjective (connected by an Ezafe), the indefinite enclitic -i (meaning "a" or "an") attaches to the very end of the entire noun phrase. So, instead of attaching to the noun (mizbân-i bâ-adab), it goes at the end of the adjective: mizbân-e bâ-adab-i.

How does the word bâ-adab literally translate to "polite"?

The prefix bâ- means "with," and adab means "manners" or "politeness." So bâ-adab literally translates to "with manners." When it takes the indefinite -i at the end for this sentence, it becomes bâ-adabi.

I don't see the Ezafe written between mizbân and bâ-adabi in the Farsi text. How do I know it's there?

The Ezafe (the "e" sound linking nouns and adjectives) is almost never written in standard Farsi script when the first word ends in a consonant. Since میزبان (mizbân) ends in the consonant 'n', you just have to know to pronounce the invisible Ezafe to connect it to the adjective: mizbân-e bâ-adabi.

Could I use 'yek' (one/a) in this sentence to say "a polite host"?

Yes, you can use the word yek before the noun (yek mizbân-e bâ-adab). However, native speakers very frequently use both yek and the -i ending together (yek mizbân-e bâ-adab-i). Any of these three ways correctly means "a polite host.".