Generally layperson (non-expert) witnesses may testify only to facts and may not testify to conclusions. The reason for this is that it is up to the jury to decide what conclusions to draw from the facts. Thus, it is improper for a witness to testify that the defendant "was driving too fast for the existing road conditions" because that is for the jury to decide. But it is hard to tell the difference between facts and conclusions since almost any label placed on some event by language is a conclusion of sorts.