Употребите Past Perfect после союза, сделав соответствующие изменения в главном предложении. 1.If they invite me, I shall go with them. 2.If I feel better, I shall go with you. 3.If she saves her money, she will be able to go on a vacation.
If they had invited me, I should have gone with them. If I had felt better I should have gone with them. If she had saved her monly she should have been able to go on a vacation.
В третьем вместо should - would нужно написать