He didn't know why to buy a computer for his son. vs He didn't know whether to buy a computer for his son.

Some examples from the web:

• Said he didn't know why Taylor ran.
• I just didn't know if I had enough money to buy a Gai-mai bao.
• And at the time, after all that happened, I still didn't know why he did it.
• Although I don't know why you'd want to buy a dump like this.
• That was different - he didn't know why he was there.
• He didn't know how to use it, but now we know why he thought he might need to.
• Audrey, I... I really couldn't handle living in this town with Nathan if you disappeared and he didn't know why.
• But he didn't know what to do.
• He moved here from D.C., and he didn't know anyone.
• But he swore he didn't know who killed my girls.
• No, he didn't know why he was doing it.
• He was so confused, so shocked, like he didn't know why - why I did this.
• Totally different right? You're ready to buy a computer from me.
• We didn't know why they made it so easy.
• I didn't know why God killed my daddy.
• At first I didn't know why.
• It frightened me, I didn't know why.
• And I didn't know why.
• Until now, I didn't know why.
• We didn't know why she was agitated.

Some examples from the web:

• The neighbors didn't know whether to be worried or titillated.
• I didn't know whether to accept or not.
• Didn't know whether to cry or to throw up.
• I didn't know whether to get red or white.
• I didn't know whether to cry or go kill myself.
• There were times when I didn't know whether to hate you or love you.
• I didn't know whether to believe any of this last night.
• I didn't know whether to stay here or go to London.
• I didn't know whether to punish you or make you pancakes.
• I didn't know whether to examine it or feed it a peanut.
• But having never chauffeured before, I didn't know whether to get him or wait in the car.
• When I found out J.R. was my half brother, I didn't know whether to celebrate or to shoot myself.
• I saw my doctor at a bus stop and I didn't know whether to say hello or not.
• It fit so perfectly that I didn't know whether to believe her or not.
• He didn't know whether he could trust Bressler and the others.
• He didn't know whether he could trust Bressler and the others.
• I didn't know whether to punish you or make you pancakes.
• I just didn't know whether to tell you or not, since it didn't change anything.
• I didn't know whether to dress for the party or for the business meeting, so I kind of dressed for both.
• When William Tell's son placed that apple on his head, he didn't know whether the arrow would pierce the fruit or his cherubic renaissance-era body.