"Ils n'ont pas réfléchi avant de parler."

Translation: They did not think before speaking..

[deactivated user]

Is there a difference in use between penser and réfléchir?

[Sitesurf]

Basically the same as between "to hear/see" vs "to listen/watch", involuntarily or voluntarily.

[Jill343161]

Why is "They do not think before they speak" incorrect?

[Sitesurf]

Because "ils n'ont pas réfléchi" is in the passé composé tense, which matches the past simple tense "they did not think" in English.