It is past because the meaning is 'she was not able to stop smiling'. I'm so happy now - I can't stop smiling! I was so happy on my wedding day - I couldn't stop smiling! I don't know what you mean by using could as a euphemism.

It really should be: The girl couldn't stop smiling because the beautiful dress FIT her so well. Fitted is not a correct conjugation.

杨浩鑫, I think I understand what you mean by "a euphemistic meaning" for this phrase. What you mean is "he can't stop smiling" has a LITERAL meaning: that he is unable to stop smiling. But this phrase isn't meant to be understood literally. It is meant to understood FIGURATIVELY: He was smiling a lot, because he was very happy. It was almost as if he had no choice and had to smile. If you are having trouble with telling the difference between literal and figurative phrases, a good way to tell the difference is to ask yourself if the sentence is possible if it is literally true. If it isn't possible, then it is probably meant to be taken figuratively. Here is an example we sometimes say: "I am so hungry, I could eat a horse." Is it possible that I could REALLY eat a horse? No, a horse is far to big for me to really be able to eat it. This is just an exaggeration. What the phrase means is "I am very hungry." Some of these figurative expressions use the word "can" or "could." But many do not. The secret to them isn't to look for any particular word. The secret to them is to ask yourself if they make sense literally. If someone says something that doesn't sound possible, they are probably using an expression that isn't meant to be taken literally. Some more examples: "I've told my son a million times to clean his room!" (this means - I've told my son MANY times) "I'm happy as a clam." (I'm very happy) "You could have knocked me over with a feather." (I was shocked when I learned something). "She was as busy as a bee." (She was very busy). "I have a green thumb." (I'm very good with plants). There are thousands of these types of expressions. Sometimes you can imagine what they mean, but sometimes you just need to ask the first time you hear them. Is your head spinning yet? (This one means "do you feel overwhelmed?" Of course, your head cannot spin).

Do you know what ლ is?:)