In short, opposite is the side opposite any angle you are taking the sin, cos or tan of. Adjacent is the side that is part of(or next to) the angle but not the hypotenuse.

Remember that Soh Cah Toa always refers to right triangles (triangles with at least one 90 degree angle).

The hypotenuse is always the side opposite the 90 degree angle.

in fact the hypotenuse is the opposite for the 90 degree angle, take a look:

therefore

And that is why sin(90) = 1, because the hypotenuse is opposite the right angle.

I hope I didn't confuse you too much.