Learn Finnish: Kirja ei ole taskussa. - The book is not in the pocket.

It comes from the noun tasku (pocket) plus the inessive ending -ssa, which means "in". Because tasku is a simple word that doesn't change its stem here, you just attach the ending directly to the end of the word.

Why does taskussa use the -ssa ending instead of -ssä?

This is due to vowel harmony. The word tasku contains the back vowels a and u. Therefore, it must take the back-vowel version of the ending, which is -ssa. If the root word only had front vowels (like y, ä, ö) or neutral vowels (i, e), it would take -ssä.

Why does the sentence use ole instead of on for "is"?

In Finnish, when you make a sentence negative, the main verb changes form. Instead of the positive on (is), you use the negative word ei followed by the verb without its personal ending. For the verb "to be", that special negative form is ole. So, "is" is on, but "is not" is ei ole.

Where are the words for "the" in this sentence?

Finnish doesn't have articles like "a" or "the". The word kirja can mean "a book" or "the book", and taskussa can mean "in a pocket" or "in the pocket". You always rely on the context to know which one makes the most sense in English.