If you do not eat Suhur or do not wake up for Fajr prayer, this is not counted as a reason for you not to fast. So you have to continue your fast. If you broke your fast, thinking that you did not have to fast, you have to fast another day to make up for it.

Imsak means to stop eating and drinking when the dawn comes. Stopping eating and drinking ten minutes before Fajr is not correct.

When a person becomes certain that the true dawn has come, he has to stop eating and drinking, and if there is food in his mouth he has to spit it out; if he does not do that, then he has invalidated his fast.

1- You should say this du`a after sighting the new moon of Ramadan and other Hijri months: ‘Allahumma ahillahu ‘alayna bi’l-yumni wa’l-iman wa’l-salamah wa’l-islam. Rabbiy wa rabbuka Allah.’ 2- There is no du`a that you should say when starting to fast each day. Rather you should simply have the intention that you are going to fast tomorrow.

If you vomit unintentionally, your fast is still valid, but if you vomit intentionally, your fast is not valid and you have to make it up later.

You do not have to take out your braces in order to fast, because nothing goes from it to the stomach. The fact that it causes you to produce more saliva does not mean that it breaks the fast.