Given that we have learned English and all the words are defined for us, is any combination of words we string together and speak/write an original thought?
Well, definition is only part of the story when it comes to meaning and use of terms is just as important. The same sentence made of the same words in the same order can have different meanings depending on use and context of use. For example "flying planes can be dangerous" may mean the act of piloting a plane, or planes in the air can be dangerous.
Or since all the definitions of words already exist, the components of thoughts that we express, would that render all expressions in English unoriginal?
No. One of the features of sentences is that even given a limited set of letters and words, there is a limitless possibility of word combinations.
Even if a person took letters from the alphabet and arranged them in a way that has not been spoken or written before, the author would then have to define what that new word or name means using the same English words that I pointed out already exist and are defined.
Well, simply saying "now" is referring to something original as right now has never existed before.
The same problem happens when a new invention is created, it generally uses a combination or modification of existing inventions.
It depends somewhat upon the scope of your descriptions. Yes, in a poetic sense, "There is nothing new under the Sun" and anything is just a different combination of particles which have been around since the beginning of spacetime, however, there is much to be said for novelty. Consider that for the U.S. Patent and Trademark Office all a patent application requires is "a novel application of an idea."
I suppose that a very small child, that does not yet know any language, could have an original thought. But would be unable to express it in any way, other than maybe appearing happy or upset without the cause known to us.
I don't really buy that. If thinking is pre-linguistic, then there is no way to express the thought, therefore, how would you ever judge originality?
Lastly, consider that status functions are something we create with language. Like the election of an official - the status function is based upon agreement about how we use our words to ordain and define the scope of powers afforded the office. We use our words such that X counts as Y in context C, e.g. the carbon fiber and ink or metals of paper and coin money count as money in the context of the economy our government secures.