Learn Farsi: درسِ نهم آسان نیست. - The ninth lesson is not easy.

It is formed by taking the base number nine, which is noh, and adding the suffix -om. This -om ending creates a "Type 1" ordinal number, which tells us the position of something in a sequence.

Why is there an 'e' sound (Ezafe) between dars and nohom?

Because nohom is acting like an adjective describing the noun dars (lesson). In Farsi, when a descriptive word like this Type 1 ordinal number comes after a noun, they must be linked by the Ezafe. So we say dars-e nohom (the ninth lesson).

Can I put the number before the noun, like we do in English?

Not with this exact word! Ordinal numbers that end in -om are placed after the noun and connected with the Ezafe. There is another type of ordinal number you might learn later that goes before the noun, but the -om type must always follow it.

I remember that ast means 'is'. Why do we use nist here?

nist is simply the negative form of ast. Instead of using a separate word for 'not' alongside 'is', Farsi combines them into this single verb. As usual in Farsi, this main verb sits at the very end of the sentence.