How to use le/la and y to refer to something previous talked about (to mean it)

Question

Can you help me to understand when to use use y ane le/la to refer to something that has been talked about before, for example
1) Tu as vu ma message ?, tu  y as  répondu ?
2) Je pense d'aller à Paris, tu le penses ?

Accepted Answer

The pronouns le/la/les refer to direct objects, whereas y refers to indirect objects preceded by à. There are lessons on this topic. If you haven't seen them yet, you'll get to them eventually.
Here's your specific examples:
Tu as répondu à mon mail ? --> Tu y as répondu ?
Your second example is a bit off. If you want to say "I'm thinking about going to Paris." You would say Je pense (à) aller à Paris --> J'y pense.
For an example using a direct object:
Je pense qu'il vient. Tu le penses aussi ? -- I think he'll come. Do you think that, too?