Learn Farsi: همه‌ی همکارهای شرکت حاضر هستند. - All the colleagues of the company are present.

The word همه describes the noun that follows it (همکارها). To link them, we use the ezafe connecting sound. Because همه ends in a silent ه (which sounds like 'e'), we must add an explicit ی to create the 'ye' sound, making it همه‌ی (hame-ye, meaning 'all of').

Since همه already means 'all', why do we still need the plural suffix ها on همکار (colleague)?

In Farsi, when you use همه to mean 'all of the' with countable things like people or objects, the noun itself must also be plural. You must say همه‌ی همکارها (literally 'all of the colleagues'), rather than leaving the noun singular.

Why is there a second ی after همکارها (colleagues)?

This sentence uses a chain of ezafe links. After linking 'all' to 'colleagues', we need to link 'colleagues' to شرکت (company) to say 'colleagues of the company'. Since the plural suffix ها ends in the vowel 'a', we add a ی for the ezafe to connect them smoothly.

Does the verb always have to be plural (هستند) when we use همه?

The verb agrees with the main noun, not just the word همه. Here, همکارها (colleagues) refers to a group of people, so we use the plural verb هستند. If you used with an uncountable noun, like (all the water), the verb would be singular.