Newbie: Declaration in AHDL

It depends what you need, but if I can give you a suggestion stop using AHDL that is a proprietary altera language and move to VHDL or Verilog.

That's most true if you have to start learning it.

To answer:

Test 1: your output is registered so it's an output of a flip flop.

Test 2: your output is pure combinatorial it's a point of a net

Test 3: I don't remember if it compiles but in that case it depend from the assignment.

Test 4: you take the point T of a netlist as the input of a flip flop whose output is your S.