But-1-ene reacts with HBr to form a saturated compound, name and draw the mechanism, then explain how three isomeric products are formed.

The mechanism is an Electrophilic Addition.  Explanation of the drawing: Draw a curly arrow from the but-1-ene double bond to the H and a curly arrow from the HBr bond to Br; forming a secondary carbocation and bromide. Lone pair on bromide attacks postive carbon and bond forms.  Three isomers: two enantiomers/ optical isomers of 2-bromobutane (major product); 1-bromobutane formed as minor product via primary carbocation.