Learn Turkish: Şirketin patronu çok zengin. - The company's boss is very rich.

The suffix -in is the genitive case, which acts like " 's " or "of" in English. Because şirket (company) ends in a consonant and its last vowel is "e", we follow vowel harmony rules and add -in to make it mean "the company's".

Why does patron have a -u at the end?

When making an "X's Y" phrase in Turkish, both parts need a suffix. While the owner gets the genitive suffix (şirketin), the owned thing gets a possessive suffix. patron gets the 3rd person singular possessive suffix -u (meaning his, her, or its boss). Together, şirketin patronu literally means "of the company, its boss".

Doesn't the "t" in şirketin need to change to a "d"?

You might expect the "t" to soften to a "d" because of the consonant mutation rule (where p, ç, t, k change when followed by a vowel). However, şirket is a loanword from Arabic. Many loanwords are exceptions to this rule and keep their original hard consonants, so it safely stays as şirketin.

Where is the word for "is" in this sentence?

Turkish doesn't use a separate word for "is" here. You simply put the subject and the adjective zengin (rich) together. While there is a formal "is" suffix (-dir), it is usually left off in everyday spoken Turkish when stating simple facts.