Conjunctions are used to link two similar grammatical structures. For example, y links two phrases of equal grammatical status, like two noun phrases (mi hermano y su amigo), while que links two verb phrases in a subordinate relationship (yo veo que tú vienes).
Here are some points to keep in mind when writing with conjunctions:
y/e:
o/u:
pero:
sino:
que:
si:
porque:
aunque: