This is an idiomatic expression, which means it follows its own rules. This is just how we use "without a word" in written English; it goes into the sentence abruptly, exactly how it is spoken. You could also call "without a word" a colloquialism. Also, it is better to say my problem came along when...., instead of my puzzle came along when.....

The subject is omitted because it is already mentioned earlier in the sentence. If it were a new sentence you would need a "he," but if you used it here it would just be repetitive. Consider this sentence: The butler pushed in the chair and he went inside. The "he" here actually makes the sentence extremely complicated, because it is impossible to tell if the sentence is referring to the butler, or some person mentioned in the sentence before. Let's add another sentence: Tom stood up. The butler pushed in the chair and he went inside. Is the "he" the butler, or Tom? Or maybe Tom is the butler? Using "he" here would actually make the sentence impossible to understand. Instead, let's use: The butler pushed in the chair and went inside. Now we obviously know it is the butler! After the "whereupon" in the sentence you have, Tom is the subject, and therefore using "he" would make the sentence possibly confusing. In the case of the sentence you have, obviously using "he" would mean "Tom," but it is basically unnecessary because we already know who the subject is in the sentence. Once you make a new sentence, you need "he."

You're most welcome! I'm other readers with similar questions will be glad you posted this. =) Keep up the good study work!!

@Weikai, Thanks a lot! ! My puzzles were blown away by your splendid, clear explanation! Thank you~

you can replace "without a word" with "quietly" -- don't get mixed up with technical terms, think of it as an adverb phrase.. it's just describing how he went inside so... Tom frowned, moved his chair backwards, and quietly went inside somewhere EX: His mother fed him some icecream, whereupon he smiled, raised his hands, and without wasting a moment hugged his mother. I personally would use a comma after "without a word" to control the reader's pace. But, that's just my flavor ;-) Hope that helps! You can comment here if you have more questions about this and I'll gladly elaborate more.