[
[["<Sentence>"], ["<NP>", "<VP>"]],
[["<Sentence>"], ["<Sentence>", "then", "<VP>"]],
[["<NP>"], ["<Det>", "<Noun>"]],
[["<VP>"], ["<Verb>", "<NP>"]],
[["<Det>"], ["the"]],
[["<Det>"], ["a"]],
[["<Noun>"], ["dog"]],
[["<Noun>"], ["penguin"]],
[["<Verb>"], ["sees"]]
]