sine is the hypotenuse divided by the adjacent side right then how can you know it by knowing just the angle

The sine ratio is the opposite side divided by the hypotenuse in a right angled triangle.

If you know 2 angles  of any triangle then you know the 3rd  because they have to add up to 180 degrees.

Any 2 triangles that have all the same angles are similar triangles.

this means that the ratio of their corresponding sides will be the same.

So

any 2 triangles with a right angle and one of the accute angles equal will be similar triangles 

so

the sine ratio for a specific acute angle will always be the same.       Capiche?