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.