Was ist der Unterschied zwischen please send me the stuff und please send me the thing ?Du kannst auch einfach Beispielsätze geben.

'Stuff' is what is called a mass noun. Mass nouns are usually things that can't be counted easily, like topics (ex. math), continuous substances (ex. water), or very small objects (ex. sand), but there are some exceptions (for some reason, furniture is a mass noun.) On the other hand, 'thing' is a count noun. These tend to be things that come in easy-to-count units (ex. card, word, block, etc.) and are much more common. When asking for something that itself is a mass noun, you would more likely say "please send me the stuff." When asking for just one of something, you would more likely say "please send me the thing." When asking for more than one of something, you could say either "please send me the stuff" or "please send me the things", but the former would probably be more common.