Download the Maple Programming Guide from here, it is a good read: 

http://www.maplesoft.com/documentation_center/

You really need to paste your exact code here, it could be that you typed a space between w and t0, just a guess.

we can obtain a result with some assumptions

(Int = int)(1/sqrt(u^n-1), u = 1 .. t) assuming n::posint, t>1

you cannot use the formal parameter as loop variable. Fix: 

exa := proc (i_formal::integer)::integer; 
local i:=i_formal;
while 1 < i do 

if mod(i, 2) = 0 then i :=i/2; print(i); else i := i+1; end if 
end do; 
end proc;

exa(10);

your function as given is implicitly defined, so you need

 plots[implicitplot3d](x^2+y^2+z^2-2^2, x = -2 .. 2, y = -2 .. 2, z = -2 .. 2, grid = [30, 30, 30], style = patchnogrid)

or use spherical coordinates:

plot3d(2,t=0..2*Pi,p=0..Pi,coords=spherical,scaling=constrained);

maybe:

Sum(C[2*i-1]*X[2*i-1], i = 1 .. 5);value(%);

or use a Library Command:

t := `Hello Bob `;
StringTools[Repeat](t, 5);

.

You may check Options->Interface and change the behaviour for Underscore Entry. Maybe this makes the difference.

.

num:=7;
mask():= [1, 2, 4][(rand(1 .. 3))()];
Bits[Xor](num, mask()); # inverts one random bit in lowest three bit positions 

Operations:=proc(xx)
local x,Square,Cubed;
Square:=evalf(xx^2);
Cubed:=evalf(xx^3);
return Square,Cubed;
end proc;

or add print commands in between:

Operations:=proc(xx)
local x,Square,Cubed;
Square:=evalf(xx^2);print(%);
Cubed:=evalf(xx^3);
end proc;

L := RandomTools[Generate](list(integer(range = 1 .. 7), 2));
x := min(L); y := max(L)

If you have any curve in space, you can set the z-value to zero for the plot:

restart:with(plots):
x := cos(t)*cos(3*t):y := sin(t)*cos(3*t):z:=2+(1/2)*sin(t):
spacecurve([[x, y, z], [x, y, 0]], t = 0 .. 2*Pi, color = red, scaling = constrained, view = [-1 .. 1, -1 .. 1, 0 .. 3], axes = framed)

simple := proc (t) return simplify(t, size), simplify(t, symbolic) end proc;

simple(sqrt(a^2*(1+a)))

other way:

simple:= proc(t) local L,i;
L:=[size,symbolic,radical];
for i in L do print(simplify(t,i)) end do;
end proc;

Explore(plot(k*x^2,x=-1..1,scaling=constrained,view=[-1..1,-1..1]),parameters=[k=-1.0..1.0]);

try simplify(Z,size);