questions

1. What actually a model is?
   1. A model is the $true$ or $false$ value for variables?
      1. Told by this video
   2. Or a set of values of variables which that evaluates the formula $true$?
      1. Told by this video
2. How to fix completeness in Propositional logic?
   1. Restricting propositional logic using clauses?
   2. Using resolution?
3. Is natural deduction and direct deduction same?
4. What is deduction theorem?
   1. is it just assuming $A$ to derive $B$ and the concluding this $A\to B$ ?