Incorrect Example: I love summer, I hate winter because I hate the cold.

This is a run-on because both ideas are independent, and pushing them together makes the sentence confusing and wordy.

You can fix run-ons by separating the two ideas with a period. Use a semicolon if the two ideas are related:

Correct Example: I love summer; I hate winter because I hate the cold.

You may also separate them with a comma and a conjunction such as “and” or “but”:

Correct Example: I love summer, but I hate winter because I hate the cold.

We would use the conjugation "and" when the the second clause of a sentence confirms the initial one.  The use of "and" underscores similarity between the clauses:

Correct Example: I love summer and I enjoy going to the beach.

Incorrect Example: He was an excellent athlete back in high school and he has gained some weight since.

We use the conjugation "but" when the second clause of a sentence partially negates the initial one.  In other words, the use of "but" links two clauses that contrast one another:

Correct Example: He was an excellent athlete back in high school but he has gained some weight since.

Incorrect Example: I love summer but I enjoy going to the beach.