Is the following sentence missing semicolons or commas? If so, where?: I wanted to sleep early last night but I just could not put down that book the final chapter is so exciting.
Yes, it is missing a comma and a semicolon.
Correction: I wanted to sleep early last night
The is in your sentence becomes a was as it is in the past tense.
Commas: These are used for separating independent clauses, or parts of a sentence that can stand as their own sentence.
Semicolons: In general, semicolons are used to separate thoughts.
Now, let's think, where could our sentence use a comma? A better question is, where do I have an independent clause?
"I wanted to sleep early last night" is an independent clause. It can stand on its own, so we can put a comma before "but".
Putting a comma before "but" is permissible, however this has been a debatable issue. This is often called the Oxford Comma.
Now let's think about semicolons. If we take a look at the sentence after our comma
"but I just could not put down that book the final chapter is so exciting"
This appears to be a run-on sentence. If we placed a comma after book, that would sound weird, namely because the clause
"but I just could not put down that book"
isn't necessarily an independent clause (usually, sentences don't start with "but").
A semicolon should go after "book", because it is separating two thoughts:
1. They couldn't put down the book
2. The final chapter is so exciting
Thus, our final sentence should be
I wanted to sleep early last night, but I just could not put down that book; the final chapter is so exciting.