Learn Finnish: Presidentti lähti palatsista valtuuskuntineen. - The president left the palace with his delegation.

It starts with the noun valtuuskunta (delegation). First, we take the plural stem valtuuskunti-. Then we add the comitative ending -ne, and finally the third-person possessive suffix -en (a variation of -nsa, meaning his or her).

The English translation says with his delegation (singular). Why does the Finnish word use the plural stem -i-?

The comitative case is unique because it always attaches to the plural stem of a noun, even if the actual meaning is singular! You have to rely on context to know if the president had one delegation or several.

Is the possessive suffix -en at the end of valtuuskuntineen mandatory?

Yes. When using the comitative case with a noun, you must always attach a possessive suffix to the very end of the word. Here, it points back to the president to show it was his delegation.

Could I just use the word kanssa (with) instead of this complicated case?

Absolutely! You could say valtuuskunnan kanssa and it would mean the exact same thing. The comitative case (-ine) is quite formal and mostly used in written Finnish, like news reports, whereas kanssa is much more common in everyday communication.