system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why some people can understand a foreign language but they can't speak it?
gpt
Because comprehension is a much easier task than production.  Comprehension just means being able to correctly recognize words and expressions and associate them to their meanings.  Producing language requires you to have a much more in depth knowledge of the vocabulary, structure, tense, etc. what words mean what and how they are used.

An analogy for this would be, imagine you're at a science conference, you go to a presentation in a field you're not familiar with, lets say astrophysics.  You'd probably be able to go there and understand what the speaker is saying, or at least get the general idea, but you couldn't produce or explain that presentation yourself, nor could you contribute anything insightful to a conversation about the material, the research and its significance with other astrophysiscs after its over.