Learn Turkish: Marketten taze portakal alıyoruz. - We are buying fresh oranges from the market.

Because taze (fresh) is an adjective, not a noun. The compound rule you are learning (adding a possessive suffix to the second word) only applies when you join two nouns together, like portakal suyu (orange juice). When using an adjective to describe a noun, you just put them next to each other: taze portakal.

The translation says 'oranges' in the plural. Why isn't it portakallar?

In Turkish, when a direct object is general or indefinite (just talking about 'fresh oranges' in general, not specific ones), it usually stays singular. Saying portakal alıyoruz is almost like saying 'we are doing orange-buying.' We translate it as plural in English because it sounds more natural.

Why is the word for 'from the market' marketten instead of marketden?

This happens because of a pronunciation rule called consonant assimilation. The word market ends in a hard, voiceless consonant (t). When you add the 'from' ending (-den), the d changes to a t to match the hard sound, making it marketten.

I learned that the verb almak means 'to take'. Why is it translated as 'buying' here?

You are right! While almak literally means 'to take', it is also the standard and most common word used for 'to buy' in everyday Turkish shopping contexts.