Note: Visit this lesson's Stroke Order subpage to see images and animations detailing how to write the following characters. Audio files of the words are linked from the pīnyīn when available. Problems listening? See media help.
Though translated as "very", Hěn [很] has a weaker meaning than it does in English. It is often added before single-syllable adjective just to enhance the rhythmic flow of the sentence. Hěn is used before the adjective in affirmative sentences, but not in negative sentences or questions. A common mistake of beginners is to insert shì [是] into adjectival sentences, but this usage is incorrect as shì can only be used to equate combinations of nouns, noun phrases and pronouns.
The particle le [了] has many different functions in Chinese, but in this case, it serves to add emphasis to the verb or adjective of the sentence. It can be seen paired with tài [太] to express excessiveness.
A sentence can be made into a question by having both affirmative and negative options together. To answer in the affirmative, the verb or adjective is repeated. (An affirmative adjective in this case is usually preceded by hěn [很] to avoid a comparative tone.) Responding in the negative is simply saying "not verb" or "not adjective".
S + V 不 V + O?
Because the bù in affirmative-negative questions is often said quickly, marking the tone on bù is not strictly necessary in their case.
In this case, you could say "她們沒有課" to mean "They don't have classes." Saying "一節課", which literally means "one period of class" would be how you emphasize that they don't have any classes. It's like saying "They don't even have one class today."