Remember "Colorless green ideas sleep furiously"? It's an example of a grammatically correct sentence....that has no discernible meaning!
So you are both right. To say, "pain is mild" is nonsense....but it is grammatically correct. So what we would like to see on DL is more sentences that are both sensible AND grammatically correct. Can I get a second to that motion?
Why is hat masculine? We just have to memorize the article with the noun to learn which gender it is. Sometimes a noun ending will give you a clue. https://www.thoughtco.com/introduction-french-nouns-1368884
If you don't remember or come across a new word, you can always look it up in a dictionary where they will usually put f for feminine or m for masculine, the n in this dictionary stands for noun. https://dictionary.reverso.net/french-english/douleur
Yes. https://dictionary.reverso.net/french-english/l%C3%A9ger When looking a word up in the dictionary though, look under the masculine form: "léger" which usually includes the feminine form "légère". Especially in this case in which the word for weight is masculine: https://dictionary.reverso.net/english-french/weight
Although the adjective will change forms depending on the gender of the noun that it is describing. "une chaise légère" , "un livre léger"