I have been told that even if all the above conditions stand, the fact that I receive a stipend from a US institution makes me a U.S. person with respect to tax...

From IRS website:“In some cases aliens are allowed to make elections which override the Green Card Test and the Substantial Presence Test, as follows:*They make...

Could you please elaborate on that? I my situation, give a treaty exists, will I be considered as resident or non resident alien with respect to my tax position...