You don't learn 汉语拼音 well...

You don't learn 汉语拼音 well, according to what you said about YU ("例如姓于的，拼写为Yu，没有拼写为Yv的").

That is the RULE of 汉语拼音: when J, Q, X and Y spell in front of "V" (stand for U with two dots on its head), the two dots shall be omitted.