TI-Basic 84 Programming/Control Flow Statements

If
If, PRGM:CTL:1 is a command that, when it's condition is true, will execute the next line of code. If the condition happens to be false, then the program will skip to the second line of code after the statement.

Else
Else, PRGM:CTL:3 is a command that, when a previous If statement is not met, will execute the next line of code. If the condition happens to be false, then the program will skip to the second line of code after the statement.

Then
Then, PRGM:CTL:2 is a command used to have a conditional statement (If or Then) execute a block of code as opposed to a single statement.