EXPLANATION A short-hand rule for commas is that we insert them in a sentence where we would pause if we were speaking the sentence aloud. While generally okay, this 'rule' can sometimes cause errors. Sometimes when we speak, we pause in places where a comma should never be used in written language. One of these places is between a subject and its verb. If you are speaking, you may need to pause to take a breath after a very long subject but you should never put a comma there if writing the sentence.
Another situation where we do need to pause when speaking is when we leave out the word that after the verb, as in:
That sentence actually is:
When we leave out words such as that, they are said to be 'understood' by the audience. That is okay when we are speaking, but when we write - and especially in formal writing - the meaning is clearer if we use full grammatical structures, so it is better to insert that than to replace it with a comma.