"I need bread for the sandwiches."
Translation:Yo necesito pan para los sándwiches.
I feel like I am still trying to master this, many years later. I am getting better, tho. Sometimes it helps me to think that if it's a future goal, it's always "para" (and generally "para" seems more commonly used. If you're torn, odds are "para" is the word you need).
This breaks it down far better, tho: