In sentence 1, the modal 'could' is used to express an ability the speaker had in the past, matching function (v). In sentence 2, 'couldn't' indicates that it was not possible to help the situation, which aligns with function (vi) impossibility. In sentence 3, 'should' is used to express a strong recommendation or sense of duty to oneself, matching function (iv) obligation. In sentence 4, 'would' is used in a conditional structure to express what the speaker expected or forecasted would happen, matching function (iii) prediction. The two extra functions, (i) promise and (ii) possibility, are not used in these contexts.