Learn Farsi: این درسِ ششم است.

In Farsi, ordinal numbers ending in -om (like ششم) always come after the noun they describe. When they are placed after the noun, they act like regular adjectives, so we must connect the noun and the number using the Ezafe vowel (-e). This gives us درسِ ششم (dars-e sheshom).

How exactly is the word ششم formed?

It is made from the regular number six, which is شش (shesh). To change a standard number into this type of ordinal number, you simply add the suffix -om. So, شش becomes ششم (sheshom).

Can I say 'the sixth lesson' by putting the number before the noun?

Yes, but you would have to use a different ending for the number. To put the number first, you must use the -omin suffix, making it ششمین درس (sheshomin dars). Notice that when the number comes first, you do not use the Ezafe connection.

Why is there a small mark under the word درس?

That mark is a 'kasra' which represents the Ezafe vowel (-e). In everyday written Farsi, this vowel is almost never written down, but you still have to pronounce it to connect the noun to its adjective. In learning materials, it is often added to help you remember to say 'dars-e sheshom' instead of just 'dars sheshom'.