Learn Russian: В санатории я читал книгу о химии. - At the sanatorium I read a book about chemistry.

Nouns ending in -ий (like санаторий) and -ия (like химия) are special. Instead of taking the standard -е in the prepositional case, their ending changes to -ии. This means санаторий becomes в санатории and химия becomes о химии.

How should I pronounce the double -ии at the end of санатории and химии?

You pronounce it as two distinct syllables, slightly elongating the sound. It sounds like a double English ee sound. Be careful not to rush it or swallow the second vowel, so it doesn't sound just like a single и.

I often get confused between в and на. Why do we use в with санатории?

A санаторий (health resort) is considered a standard enclosed building or institution in Russian, so it takes в (in/at). You generally use на for events, open spaces, or specific exceptions like на почте (at the post office) or на вокзале (at the train station).

Why did the speaker use the imperfective verb читал instead of the perfective прочитал?