We defined the ship39;s positions.They received a radiogram.The sea was rough.There

1) We defined the ships positions. - We did not define the ships positions.
2) They received a radiogram. - They did not receive a radiogram.
3) The sea was rough. - The sea was not rough.
4) There were gales over the sea. - There were not gales over the sea.
В первых 2-ух предложениях глагол did сменяет окончание -ed в глаголах прошедшего медли.
В 3-ем и четвертом предложениях мы прибавляем частичку not для того чтобы обозначить то, что предложение отрицательное.