The use of the word 'the' in English indicates a singularity of the noun. This sentence indicates that 'there is only one ambulance (in the whole world?) and I need it'. 'An' ambulance does not have this wierdness
Well, maybe there was a discussion earlier about an ambulance and the fact that it may be needed, so 'the' would be kind of understandable... but I definitely agree that 'an' should be the default translation.
Why isn't the accusative of the word "karetka" -> "karetkę" instead of "karetki"?
Words ending in ...k... have their own rules for grammar. If you're hitting your head against your phone reading this: Get used to it :)
The Accusative is "karetkę" indeed. But this is Genitive. The verb "potrzebować" takes Genitive.
It is quite a common mistake among Polish natives to use Accusative instead, though. But it's a mistake nevertheless.
Oh, yeah. I just learned about this particularly nuance :) some verbs just use genitive ... just because. If you happen to have a link to said list that could be useful. Thanks again