"How come the baby is dirty?"
Translation:Mbona mtoto ni mchafu?
Further to Ben's comment, another way to look at "how come" is "how came this to be?"
Why is the baby dirty?
How did the baby come to be dirty?
Personally, I think in this case mbona mtoto ni mchafu is closer in feel to "what on earth happened to the baby??" ;)
Yep. It means "Why is the baby dirty?"
"How come..." is a bit informal and can only be used to ask about a cause in the past of the action in question. ("What ... for?" can be used to ask about a goal in the future. "Why" can be used for both.)
If you've never seen "How come" before, I can understand your confusion! It's a weird sentence if you consider the words individually.
The tips and notes rather misleadingly say
Mbona? How come? (stronger meaning than ‘why?’)
I can see why they want to use a different term than "why" for translating mbona, but I'm in agreement with you that "how come" is a bit informal.
So I could argue the English translation, while indeed correct English, hasn't the right tone. Mbona is "why", just more emphatic than kwa nini.