When do you say that two triangles have the same shape?

well, they could be congruent, there are some rules for congruency, to be congruent two triangles have, ASA-two angles the same with a side length between them. SAS-two side lengths the same and a same angle between them. SSS-all 3 side lengths the same. RHS-if the triangles are right angles ,and the hypotenuse are the same. ;or they could be mathmatically similar. :)