Learn French: Ils n'étudient pas à minuit. - They do not study at midnight.

In French, just like in English, we use special words for 12:00. Use midi for noon and minuit for midnight instead of saying douze heures. Notice that we use the preposition à to mean "at" that specific time.

Why is it n'étudient pas instead of ne étudient pas?

The negative word ne drops its e and gets an apostrophe because the verb étudient starts with a vowel. This makes the phrase flow better when spoken.

How do you pronounce the -ent at the end of étudient?

You don't! The -ent ending on verbs for ils and elles (they) is completely silent. You pronounce étudient exactly like the singular form étudie.

Can I use à for general parts of the day, like "in the morning"?

You only use à for specific clock times, like à minuit (at midnight) or à trois heures (at three o'clock). For general parts of the day, French usually doesn't use a preposition, so "in the morning" is simply le matin.