Very useful article.

However, the beginning of this article made me little confused.

This must be an age thing, I'm a "little" older than the author, BTW in Brit speak "little" older means "a lot".

635 is six hundred and thirty five.  There is no need of a hyphen because there is no undercertainty about what number you mean.  I suspect there has been an Americanisation of the British education system to bring in the spurious hyphen on the back of the internet.

Great article BTW.

Very nicely done! Your explanations were thorough and also, I imagine, very helpful to English students. It's rare to find an article with this level of depth and quality of writing.

I did notice the example tables are a little squashed in the headers; perhaps there is a way to add a little space so they're easier to read?

I don't think you need the dash between the 10's and 1's if it's not directly being used as an adjective for something else (such as dollars, euros, or whatever else you're counting), but perhaps that is a quirk of American English (thirty five on its own, versus thirty-five dollars).

Also, in American English we do actually say the "and" between the 100's and the 10's if the 1's are included; it is just shortened to an 'n' with a glottal stop (like how "mountain" becomes "moun'n"). "One hundred 'n twenty five," for instance. It's just very quiet. The only time we don't is if we're counting, and we'll say either "one hundred 'n twenty five", or "one twenty-five", leaving out the "hundred".

Again, thank you again for putting this together. You put a lot of effort into it and it is truly quality writing :) Cheers from across the pond!