"These clothes are so pretty!"
Translation:¡Esta ropa es tan bonita!
It's not exactly plural, rather it's an uncountable noun. Like water is, for example. You can be talking about a lot of water, but you always say "water is..." not "water are..." Think about it like the word 'clothing' instead of 'clothes' and that might help because clothing is uncountable in English the way that ropa is in Spanish. "This clothing is so pretty" not "these clothing are so pretty."