If a fasting person swallows his saliva, that does not invalidate his fast even if there is a lot of it.

There is nothing wrong with simply rinsing out the mouth while fasting so long as the water does not enter the throat of the fasting person.

There is no harm for a fasting person to taste food as needed. If they accidentally swallow it, there is no sin, and they can continue their fast.

There is nothing wrong with a man playing with his wife, or a wife with her husband, by saying words while fasting, on the condition that there is no danger of either of them climaxing. It is permissible to kiss and play with one’s wife, if one is sure that there is no risk of reaching climax.

There is nothing wrong with cutting nails or shaving the pubic hair while fasting.

You should not delay ghusl after having sexual intercourse with your wife in Ramadan so that you can pray Fajr with the congregation in the mosque.

You have to try hard to give up this habit, and take your thumb out of your mouth every time you realize that you are sucking your thumb. Whatever happens because of forgetfulness will not affect your fast.