"The girl is too young for that."
Translation:La niña es muy joven para eso.
Before you can choose between por and para you need to know what aisle of the Spanish supermarket you're on.
These are the only five confusing aisles, the ones that contain both por and para:
- Purpose (para) vs. Reason (por) <= In your sentence we're talking about a purpose (to be able to do that)
- Normal recipient (para) vs. Favor recipient (por)
- Moment in time (para) vs. Amount of time (por)
- Destination (para) vs. Route (por)
- Opinion (para) vs. Indifference (por)
If you're not in any of these five aisles (or if you are, but para doesn't fit), use por.
I go deep into the usage of por and para in this post: http://itsnachotime.com/blog/por-para
I too have trouble figuring out when to use por vs para. The rules I found suggest por is more often used. The closest rule I could find relating to this question was Rule: to express a contrast from what is expected; Model: Para un niño lee muy bien.(For a child, he reads very well.) I will no doubt have to refer to the lists of uses in the future.
I was always told that 'para' would be translated as '(intended) for'. 'Intended' wouldn't go in the translation but if you could translate, for example, "This is for him" then could you (without changing the meaning) also say "This is intended for him"? then use 'para' else use 'por'.
Over time (with lots of use) it will become second nature and you won't have to translate it in your head.
No sweetie, in this case you would have to use 'demasiado' as it means too- as in 'too young' or ' too old', just like the translation and más means more. However más would only be applicable if the question was comparing two things, for example, 'my sister is younger than me'( Mi hermana es más joven que yo). Also, another important grammatical point is that, más is usually followed by que- just like the example.