Learn Italian: Questo è il diciassettesimo libro che leggo in questa lingua. - This is the seventeenth book I have read in this language.

The Italian word for 17 is diciassette. To form ordinal numbers from 11 upwards, you drop the final vowel of the original number and add the suffix -esimo. So, diciassette drops its final -e to become diciassettesimo.

Are there any tricky spelling rules to remember for diciassettesimo?

Just remember to keep all the double consonants from the original number! Because diciassette already has a double s and a double t, you must keep both of those doubles when adding the -esimo suffix.

Why is the present tense leggo translated as "I have read" instead of "I read"?

In English, we usually use the present perfect ("I have read") when counting experiences up to the present moment. In Italian, however, it is very common to just use the present tense (leggo) in relative clauses like this one, literally saying "This is the seventeenth book that I read."

Why do we use questo at the beginning of the sentence but questa at the end?