Learn Farsi: تاکسی گران است.

In spoken Farsi, the -ān sound often changes to -un, making گران (gerān) sound like گرون (gerun). Additionally, the formal verb است (ast) usually merges with the previous word and sounds like an -e at the end. So, a native speaker would actually say تاکسی گرونه (tāksi gerune).

The English translation says 'The taxi', but there is no specific word for 'the' in the Farsi sentence. How do we know it is definite?

Farsi lacks a direct equivalent for the definite article 'the'. A noun standing alone like تاکسی is naturally treated as definite if the context implies a specific taxi. If you wanted to say 'a taxi' in general, you would add the indefinite suffix to make it تاکسی‌ای (tāksi-i).

What is the exact opposite of گران that I could use to negotiate the fare?

The opposite is ارزان (arzān), which means cheap. Following the same spoken rule, it is pronounced ارزون (arzun) in daily conversation. You might tell the driver ارزون‌تر حساب کن (calculate it cheaper) if you are haggling.