Learn Russian: Не лги мне, я всё видел. - Do not lie to me, I saw everything.

In Russian, negative imperatives (telling someone not to do something) almost always use the imperfective aspect. This expresses a general prohibition ('don't do this action'). If you used the perfective with не, it would sound like a warning against doing something accidentally, which doesn't make sense for a deliberate action like lying.

I usually hear the verb врать used for 'to lie'. What is the difference between лгать (used here) and врать?

Both mean 'to lie', but врать is more colloquial and commonly used in everyday, casual speech. лгать (where лги comes from) is slightly more formal, serious, or dramatic. Saying не ври мне would mean exactly the same thing but sound just a bit more conversational.

Why is the pronoun мне in the dative case?

Verbs of communication, including лгать (to lie), говорить (to tell), and отвечать (to answer), require the dative case to indicate the person being spoken to. You are literally saying 'Do not tell lies to me'.

Why does the sentence use всё instead of весь or все?

всё is the neuter singular form, which functions here as the standalone pronoun meaning 'everything'. все (plural) means 'everyone' or 'all (people)', and весь (masculine) would be used as an adjective modifying a specific masculine noun, like in весь день (all day).