"Dà due esempi principali."
Dà due esempi principali has
dà which is 3rd person singular leading to 'He gives two main examples'. But
dà is also the 2nd person imperative so it seems that 'Give two main examples' should be accepted as well. But perhaps Italians would not likely intend that meaning even if it is correct?
Actually the imperative should be da', contraction of dai which would be indistinguishable from the present; dà is allowed as well but not so common. According to the Treccani dictionary using "dà" would force a syntactic doubling (raddoppiamento [fono]sintattico), i.e. the next word should be pronounced as if the first letter was doubled. Never heard that in real life though.