If you want to say this type of pants is fashion now, you can say: "Esta es la última moda en pantalones"

¡Hola Shirley! It's a pleasure to meet you. "Un pantalón" it's not wrong at all. This word is actually pretty interesting, you can choose to say either, "un pantalón" or "unos pantalones" to refer to a singular specific pair of pants. Even if you're not refering to more than 1 pair of pants, you can still say "unos pantalones". Now, "este es la ultimo moda de los pantalones" is wrong because the word "moda" (fashion, trend) is feminine and the adjectives in the sentence need to be feminine as well. Having said that, the correct way to express it would be: "estA es la ultimA moda de los pantalones". Hope this clarifies your doubts. If you have any more questions, please don't hesitate to contact me. Hope you have an awesome evening Shirley! Gabriel.

你好Shirley! 我们说一条裤子,就像我们说一双鞋. 如果你说 "我要穿这条裤子", 你可以说 " voy a usar este pantalón" ( 单数), 但是你说 " es la última moda en pantalones " 你应该用复数的 (我们使用的复数术语，因为裤子上有两条腿) 我希望我的解释已经明确, 我的汉语不太好.

when you say: "quería ponerme unos pantalones negros". you are talking about one object. similar words are: tijeras, pinzas, gafas. And if you say: "me gusta ese pantalón" is correct too. "esta es la ultima moda de los pantalones"