FormBox[boxes, form] is a low-level box construct which displays as boxes but specifies that rules associated with form should be used to interpret boxes on input.

SyntaxForm is an option for InterpretationBox and TagBox objects that specifies the precedence level to use when the InterpretationBox or TagBox is considered as an operator.

AccountingForm[expr] prints with all numbers in expr given in standard accounting notation. AccountingForm[expr, n] prints with numbers given to n-digit precision.

StringForm["controlstring", expr_1, ...] prints as the text of the controlstring, with the printed forms of the expr_i embedded.

NumberForm[expr, n] prints with approximate real numbers in expr given to n-digit precision.

MathMLForm[expr] prints as a MathML form of expr.

EngineeringForm[expr] prints with all real numbers in expr given in engineering notation. EngineeringForm[expr, n] prints with numbers given to n-digit precision.

PrecedenceForm[expr, prec] prints with expr parenthesized as it would be if it contained an operator with precedence prec.

Since lists are just a particular kind of expression, it will come as no surprise that you can refer to parts of any expression much as you refer to parts of a list. This ...

TreeForm[expr] displays expr as a tree with different levels at different depths. TreeForm[expr, n] displays expr as a tree only down to level n.