How did the alliances in place before world war i affect the war?

These alliances essentially decided who was fighting who. Countries who were allies with each other ended up fighting together. For example, the spark that began WWI was the assassination of Archduke Franz Ferdinand of Austria; after his assassination, it was essentially as follows:
England joined the war because Germany attacked Belgium trying to get to France because France was allies with Russia who was allies with Serbia who wanted land from Austria-Hungary, which is why Archduke Franz Ferdinand was killed. Then the US and other countries joined because of threats to their allies and threats from other countries and so on. So allegiances played a big part in not only where the war was being fought but who it was fought by. I hope this helped!