Theory:

Simple and Compound Sentences

A simple sentence consists of only one clause, which means it only has one subject, one verb, and doesn't use any phrases or additional clauses for extra detail, other that simple adjectives:

I have a dog.

A SENTENCE must start with capital letter, and finish with a full stop, a question mark if it is a question, or an exclamation mark if it is an exclamation.

A COMPOUND SENTENCE contains two or more SIMPLE SENTENCES joined by a conjunction.

I like rugby, but I prefer tennis.

The most common conjunctions are AND, BUT, and OR.

Note: COMPOUND SENTENCES often need COMMAS, and they are commonly used mistakenly. The simple rule is to use a comma before the conjunction:

I like rugby, but I prefer tennis.

It is not correct to join two simple sentences with a comma but without a conjunction.

He was the best player, he lost the game. (incorrect)

He was the best player, but he lost the game.(correct)