The fasting person is enjoined to fear Allah and to do what He has commanded, and to avoid what He has forbidden. Talking to the opposite gender does not break the fast but it takes away from the reward of fasting.

If you are able to expel the food stuck between the teeth but you do not do so and you swallow it, then you have invalidated your fast. If you swallow the food stuck between the teeth involuntarily, then your fast is valid.

Smoking is haram whether in Ramadan or outside Ramadan. Smoking invalidates the fasting because the smoke contains particles that reach the stomach

If someone eats or drinks after Fajr, mistakenly believing Fajr has not arrived, their fast remains valid. Scholars affirm that unintentional consumption does not break the fast, based on Quranic evidence and Hadith.

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.

Yes, snus breaks the fast because when it is placed in the mouth it dissolves and some particles of it enter the stomach with the saliva.