Learn Dutch: De vakbond eist een hoger salaris voor de arbeiders. - The trade union demands a higher salary for the workers.

Salaris is a neuter (het) word. When a het-word follows the indefinite article een, the adjective modifying it does not get an -e at the end. Here, we are using the comparative form of hoog (high), which is hoger. Because of the rule, it remains hoger without the extra -e. If we used the definite article, it would be het hogere salaris.

What is the exact nuance of eist compared to just asking for a raise?

Eisen means to demand or require something, usually forcefully or as a hard condition. It is much stronger than vragen (to ask) or verzoeken (to request). In the context of a vakbond (trade union), it implies that industrial action, like a strike, might follow if the demand is not met.

Could I say ...eist een hoger salaris van de arbeiders instead of voor?

You could, but it changes the meaning completely! The phrase iets eisen van iemand means to demand something from someone (so the workers would have to pay the salary). Using voor (for) means the union is demanding the higher salary on behalf of or for the benefit of the workers.

Why does arbeiders get an -s plural instead of ?