Negation Across Tenses: lā, lam, lan

Arabic uses a different word for not for each time, and two of them change the verb's ending:

• لا + present = does not: هم لا يشربون القهوة (they don't drink coffee). ما + present is also correct.
• لم + short present form = did not: لم يذهبوا (they didn't go). ما + past says the same: ما ذهبوا.
• لن + short present form = will not: لن يذهبوا (they won't go).

On the masculine plural, dual and you (f.) forms you can see the difference in writing: after لم and لن the final ن drops (يذهبون → يذهبوا, يسافران → يسافرا). So if the verb still ends in ن, choose لا. If the ن is gone, the time word decides: أمس or الماضي → لم, غدا or القادم → لن.