A comprises B and C, or A is composed of B and C. Constitute means to form. The party is composed of politicians and journalists, comprises 15 women and 10 men, and constitutes a new direction in the relationship between politicians and the media. Following the rails, we soon came to a station , but After the rain, we were able to leave the shelter . The Americans say presently when they mean immediately . This usage can be very confusing, and should be avoided even by Americans.
Arbitration is used when negotiations are unsuccessful .
This implies that negotiations sometimes fail. Arbitration may be used if negotiations are unsuccessful . This, that negotiation may or may not fail. Arbitration may be used where negotiations are unsuccessful. And this, that negotiations will be successful in some places and unsuccessful in others.