"I need one hundred dollars to buy clothes."
Translation:Necesito cien dólares para comprar ropa.
Why is it wrong to use "la ropa"? It seems spanish often uses a definite article when english does not, so why not here?
"para" is a preposition often used for expressing the purpose of something, you can think of it sometimes as "in order to". In this sentence, the english "to" is expressing the purpose for which the money is needed, and thus "para" is needed in the translation to convey the same meaning.
so in spanish when we talk about something like "buying clothes" we use "ropa" instead of "ropas"?
You could compare it to an English word like "clothing" or, if you think of a completely different word that "works"the same in Enlgish : money. They are words that have no plural, but indicate a general idea.
How can you tell when it should be "quiero" and when it should be "requiero"?
"querer" is really more like "want".
Use "necesitar" for "need".
"requerir" is more like "demand".
In informal communication we may hear "I want", "I need", "I have to have", "I've got to have" and undoubtedly other similar phrases used to indicate the intensity of the demand.
I am sure Spanish works in much the same way but we will only gain a sense of which is appropriate later on when we have mastered the basic vocabulary and grammar and have moved on to recognising which phrases are best in which context. This is too advanced for Duolingo's simple exercises.