The three short vowel marks

بَ بِ بُ

ب with a short line above is ba. The same letter with a line below is bi. With a small loop above it is bu. The letter did not change. Only the mark did.

These marks are called fatha, kasra and damma. The first book teaches them after the letter shapes, because the shape tells you which letter it is and the mark tells you the vowel.