Shall and will can be interchanged.

“Are going to” is slightly different. Still future yes, but is usually used to indicate intention rather than decisions.

“I will shit soon” and “I am going to shit” both mean basically the same thing but the connotations of those sentences are slightly different.

Nouns don’t need tenses.

Shit in the future and shit in the past.

Similar to many verbs that need an object, you add “will” to the present tense.

He will run. She will jump. You will shit.