What happens if I convert from H4 to F1 before joining Masters & if couldn't join the university?

What happens if I convert from H4 to F1 before joining Masters and if I couldn’t join the university.What will be my visa status? Will it retain as H4?

Once you convert or your H4 to F1 is approved, you have to fulfill the requirements needed to maintain F1 status (SEVIS records etc.). If you fail, your F1 will be cancelled by the university and you will go out of status hence forth, making your stay in USA illegal.

Once they cancel, within the grace period, you can apply fro F1 to H4 conversion and have it approved or you can go out of the country and enter in on H4 to continue to live in USA on H4 status.

fine.How much will be the grace period duration?only after grace period duration status would become illegal right?

Yes. Within your F1 period grace period expires, you should have applied for F1 to H4 change of status in order to remain in status within USA.

You can always travel out of USA and enter back on H4 to make it simple.